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<pre id='vimCodeElement'>
<span class="Comment"># A doubly linked list permits bidirectional traversal.</span>

<span class="muData">container</span> duplex-list:_elem [
  value:_elem
  next:&amp;:duplex-list:_elem
  prev:&amp;:duplex-list:_elem
]

<span class="Comment"># should I say in/contained-in:result, allow ingredients to refer to products?</span>
<span class="muRecipe">def</span> push x:_elem, in:&amp;:duplex-list:_elem<span class="muRecipe"> -&gt; </span>in:&amp;:duplex-list:_elem [
  <span class="Constant">local-scope</span>
  <span class="Constant">load-ingredients</span>
  result:&amp;:duplex-list:_elem<span class="Special"> &lt;- </span>new <span class="Delimiter">{</span>(duplex-list _elem): type<span class="Delimiter">}</span>
  *result<span class="Special"> &lt;- </span>merge x, in, <span class="Constant">0</span>
  <span class="Delimiter">{</span>
    <span class="muControl">break-unless</span> in
    *in<span class="Special"> &lt;- </span>put *in, <span class="Constant">prev:offset</span>, result
  <span class="Delimiter">}</span>
  <span class="muControl">return</span> result  <span class="Comment"># needed explicitly because we need to replace 'in' with 'result'</span>
]

<span class="muRecipe">def</span> first in:&amp;:duplex-list:_elem<span class="muRecipe"> -&gt; </span>result:_elem [
  <span class="Constant">local-scope</span>
  <span class="Constant">load-ingredients</span>
  <span class="muControl">return-unless</span> in, <span class="Constant">0</span>
  result<span class="Special"> &lt;- </span>get *in, <span class="Constant">value:offset</span>
]

<span class="muRecipe">def</span> next in:&amp;:duplex-list:_elem<span class="muRecipe"> -&gt; </span>result:&amp;:duplex-list:_elem/contained-in:in [
  <span class="Constant">local-scope</span>
  <span class="Constant">load-ingredients</span>
  <span class="muControl">return-unless</span> in, <span class="Constant">0</span>
  result<span class="Special"> &lt;- </span>get *in, <span class="Constant">next:offset</span>
]

<span class="muRecipe">def</span> prev in:&amp;:duplex-list:_elem<span class="muRecipe"> -&gt; </span>result:&amp;:duplex-list:_elem/contained-in:in [
  <span class="Constant">local-scope</span>
  <span class="Constant">load-ingredients</span>
  <span class="muControl">return-unless</span> in, <span class="Constant">0</span>
  result<span class="Special"> &lt;- </span>get *in, <span class="Constant">prev:offset</span>
  <span class="muControl">return</span> result
]

<span class="muScenario">scenario</span> duplex-list-handling [
  run [
    <span class="Constant">local-scope</span>
    <span class="Comment"># reserve locations 0-9 to check for missing null check</span>
    <span class="Constant">10</span>:num/<span class="Special">raw &lt;- </span>copy <span class="Constant">34</span>
    <span class="Constant">11</span>:num/<span class="Special">raw &lt;- </span>copy <span class="Constant">35</span>
    list:&amp;:duplex-list:char<span class="Special"> &lt;- </span>push <span class="Constant">3</span>, <span class="Constant">0</span>
    list<span class="Special"> &lt;- </span>push <span class="Constant">4</span>, list
    list<span class="Special"> &lt;- </span>push <span class="Constant">5</span>, list
    list2:&amp;:duplex-list:char<span class="Special"> &lt;- </span>copy list
    <span class="Constant">20</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>next list2
    <span class="Constant">21</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>next list2
    <span class="Constant">22</span>:char/<span class="Special">raw &lt;- </span>first list2
    <span class="Constant">30</span>:&amp;:duplex-list:char/<span class="Special">raw &lt;- </span>next list2
    <span class="Constant">31</span>:char/<span class="Special">raw &lt;- </span>first <span class="Constant">30</span>:&amp;:duplex-list:char/<span class="Special">raw</span>
    <span class="Constant">32</span>:&amp;:duplex-list:char/<span class="Special">raw &lt;- </span>next <span class="Constant">30</span>:&amp;:duplex-list:char/<span class="Special">raw</span>
    <span class="Constant">33</span>:&amp;:duplex-list:char/<span class="Special">raw &lt;- </span>prev <span class="Constant">30</span>:&amp;:duplex-list:char/<span class="Special">raw</span>
    list2<span class="Special"> &lt;- </span>prev list2
    <span class="Constant">40</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>prev list2
    <span class="Constant">41</span>:char/<span class="Special">raw &lt;- </span>first list2
    <span class="Constant">50</span>:bool/<span class="Special">raw &lt;- </span>equal list, list2
  ]
  memory-should-contain [
    <span class="Constant">0</span><span class="Special"> &lt;- </span><span class="Constant">0</span>  <span class="Comment"># no modifications to null pointers</span>
    <span class="Constant">10</span><span class="Special"> &lt;- </span><span class="Constant">34</span>
    <span class="Constant">11</span><span class="Special"> &lt;- </span><span class="Constant">35</span>
    <span class="Constant">20</span><span class="Special"> &lt;- </span><span class="Constant">5</span>  <span class="Comment"># scanning next</span>
    <span class="Constant">21</span><span class="Special"> &lt;- </span><span class="Constant">4</span>
    <span class="Constant">22</span><span class="Special"> &lt;- </span><span class="Constant">3</span>
    <span class="Constant">30</span><span class="Special"> &lt;- </span><span class="Constant">0</span>  <span class="Comment"># null</span>
    <span class="Constant">31</span><span class="Special"> &lt;- </span><span class="Constant">0</span>  <span class="Comment"># first of null</span>
    <span class="Constant">32</span><span class="Special"> &lt;- </span><span class="Constant">0</span>  <span class="Comment"># next of null</span>
    <span class="Constant">33</span><span class="Special"> &lt;- </span><span class="Constant">0</span>  <span class="Comment"># prev of null</span>
    <span class="Constant">40</span><span class="Special"> &lt;- </span><span class="Constant">4</span>  <span class="Comment"># then start scanning prev</span>
    <span class="Constant">41</span><span class="Special"> &lt;- </span><span class="Constant">5</span>
    <span class="Constant">50</span><span class="Special"> &lt;- </span><span class="Constant">1</span>  <span class="Comment"># list back at start</span>
  ]
]

<span class="Comment"># insert 'x' after 'in'</span>
<span class="muRecipe">def</span> insert x:_elem, in:&amp;:duplex-list:_elem<span class="muRecipe"> -&gt; </span>in:&amp;:duplex-list:_elem [
  <span class="Constant">local-scope</span>
  <span class="Constant">load-ingredients</span>
  new-node:&amp;:duplex-list:_elem<span class="Special"> &lt;- </span>new <span class="Delimiter">{</span>(duplex-list _elem): type<span class="Delimiter">}</span>
  *new-node<span class="Special"> &lt;- </span>put *new-node, <span class="Constant">value:offset</span>, x
  <span class="Comment"># save old next before changing it</span>
  next-node:&amp;:duplex-list:_elem<span class="Special"> &lt;- </span>get *in, <span class="Constant">next:offset</span>
  *in<span class="Special"> &lt;- </span>put *in, <span class="Constant">next:offset</span>, new-node
  *new-node<span class="Special"> &lt;- </span>put *new-node, <span class="Constant">prev:offset</span>, in
  *new-node<span class="Special"> &lt;- </span>put *new-node, <span class="Constant">next:offset</span>, next-node
  <span class="muControl">return-unless</span> next-node
  *next-node<span class="Special"> &lt;- </span>put *next-node, <span class="Constant">prev:offset</span>, new-node
]

<span class="muScenario">scenario</span> inserting-into-duplex-list [
  run [
    <span class="Constant">local-scope</span>
    list:&amp;:duplex-list:char<span class="Special"> &lt;- </span>push <span class="Constant">3</span>, <span class="Constant">0</span>
    list<span class="Special"> &lt;- </span>push <span class="Constant">4</span>, list
    list<span class="Special"> &lt;- </span>push <span class="Constant">5</span>, list
    list2:&amp;:duplex-list:char<span class="Special"> &lt;- </span>next list  <span class="Comment"># inside list</span>
    list2<span class="Special"> &lt;- </span>insert <span class="Constant">6</span>, list2
    <span class="Comment"># check structure like before</span>
    list2<span class="Special"> &lt;- </span>copy list
    <span class="Constant">10</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>next list2
    <span class="Constant">11</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>next list2
    <span class="Constant">12</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>next list2
    <span class="Constant">13</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>prev list2
    <span class="Constant">20</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>prev list2
    <span class="Constant">21</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>prev list2
    <span class="Constant">22</span>:char/<span class="Special">raw &lt;- </span>first list2
    <span class="Constant">30</span>:bool/<span class="Special">raw &lt;- </span>equal list, list2
  ]
  memory-should-contain [
    <span class="Constant">10</span><span class="Special"> &lt;- </span><span class="Constant">5</span>  <span class="Comment"># scanning next</span>
    <span class="Constant">11</span><span class="Special"> &lt;- </span><span class="Constant">4</span>
    <span class="Constant">12</span><span class="Special"> &lt;- </span><span class="Constant">6</span>  <span class="Comment"># inserted element</span>
    <span class="Constant">13</span><span class="Special"> &lt;- </span><span class="Constant">3</span>
    <span class="Constant">20</span><span class="Special"> &lt;- </span><span class="Constant">6</span>  <span class="Comment"># then prev</span>
    <span class="Constant">21</span><span class="Special"> &lt;- </span><span class="Constant">4</span>
    <span class="Constant">22</span><span class="Special"> &lt;- </span><span class="Constant">5</span>
    <span class="Constant">30</span><span class="Special"> &lt;- </span><span class="Constant">1</span>  <span class="Comment"># list back at start</span>
  ]
]

<span class="muScenario">scenario</span> inserting-at-end-of-duplex-list [
  run [
    <span class="Constant">local-scope</span>
    list:&amp;:duplex-list:char<span class="Special"> &lt;- </span>push <span class="Constant">3</span>, <span class="Constant">0</span>
    list<span class="Special"> &lt;- </span>push <span class="Constant">4</span>, list
    list<span class="Special"> &lt;- </span>push <span class="Constant">5</span>, list
    list2:&amp;:duplex-list:char<span class="Special"> &lt;- </span>next list  <span class="Comment"># inside list</span>
    list2<span class="Special"> &lt;- </span>next list2  <span class="Comment"># now at end of list</span>
    list2<span class="Special"> &lt;- </span>insert <span class="Constant">6</span>, list2
    <span class="Comment"># check structure like before</span>
    list2<span class="Special"> &lt;- </span>copy list
    <span class="Constant">10</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>next list2
    <span class="Constant">11</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>next list2
    <span class="Constant">12</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>next list2
    <span class="Constant">13</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>prev list2
    <span class="Constant">20</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>prev list2
    <span class="Constant">21</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>prev list2
    <span class="Constant">22</span>:char/<span class="Special">raw &lt;- </span>first list2
    <span class="Constant">30</span>:bool/<span class="Special">raw &lt;- </span>equal list, list2
  ]
  memory-should-contain [
    <span class="Constant">10</span><span class="Special"> &lt;- </span><span class="Constant">5</span>  <span class="Comment"># scanning next</span>
    <span class="Constant">11</span><span class="Special"> &lt;- </span><span class="Constant">4</span>
    <span class="Constant">12</span><span class="Special"> &lt;- </span><span class="Constant">3</span>
    <span class="Constant">13</span><span class="Special"> &lt;- </span><span class="Constant">6</span>  <span class="Comment"># inserted element</span>
    <span class="Constant">20</span><span class="Special"> &lt;- </span><span class="Constant">3</span>  <span class="Comment"># then prev</span>
    <span class="Constant">21</span><span class="Special"> &lt;- </span><span class="Constant">4</span>
    <span class="Constant">22</span><span class="Special"> &lt;- </span><span class="Constant">5</span>
    <span class="Constant">30</span><span class="Special"> &lt;- </span><span class="Constant">1</span>  <span class="Comment"># list back at start</span>
  ]
]

<span class="muScenario">scenario</span> inserting-after-start-of-duplex-list [
  run [
    <span class="Constant">local-scope</span>
    list:&amp;:duplex-list:char<span class="Special"> &lt;- </span>push <span class="Constant">3</span>, <span class="Constant">0</span>
    list<span class="Special"> &lt;- </span>push <span class="Constant">4</span>, list
    list<span class="Special"> &lt;- </span>push <span class="Constant">5</span>, list
    list<span class="Special"> &lt;- </span>insert <span class="Constant">6</span>, list
    <span class="Comment"># check structure like before</span>
    list2:&amp;:duplex-list:char<span class="Special"> &lt;- </span>copy list
    <span class="Constant">10</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>next list2
    <span class="Constant">11</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>next list2
    <span class="Constant">12</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>next list2
    <span class="Constant">13</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>prev list2
    <span class="Constant">20</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>prev list2
    <span class="Constant">21</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>prev list2
    <span class="Constant">22</span>:char/<span class="Special">raw &lt;- </span>first list2
    <span class="Constant">30</span>:bool/<span class="Special">raw &lt;- </span>equal list, list2
  ]
  memory-should-contain [
    <span class="Constant">10</span><span class="Special"> &lt;- </span><span class="Constant">5</span>  <span class="Comment"># scanning next</span>
    <span class="Constant">11</span><span class="Special"> &lt;- </span><span class="Constant">6</span>  <span class="Comment"># inserted element</span>
    <span class="Constant">12</span><span class="Special"> &lt;- </span><span class="Constant">4</span>
    <span class="Constant">13</span><span class="Special"> &lt;- </span><span class="Constant">3</span>
    <span class="Constant">20</span><span class="Special"> &lt;- </span><span class="Constant">4</span>  <span class="Comment"># then prev</span>
    <span class="Constant">21</span><span class="Special"> &lt;- </span><span class="Constant">6</span>
    <span class="Constant">22</span><span class="Special"> &lt;- </span><span class="Constant">5</span>
    <span class="Constant">30</span><span class="Special"> &lt;- </span><span class="Constant">1</span>  <span class="Comment"># list back at start</span>
  ]
]

<span class="Comment"># remove 'x' from its surrounding list 'in'</span>
<span class="Comment">#</span>
<span class="Comment"># Returns null if and only if list is empty. Beware: in that case any other</span>
<span class="Comment"># pointers to the head are now invalid.</span>
<span class="muRecipe">def</span> remove x:&amp;:duplex-list:_elem/contained-in:in, in:&amp;:duplex-list:_elem<span class="muRecipe"> -&gt; </span>in:&amp;:duplex-list:_elem [
  <span class="Constant">local-scope</span>
  <span class="Constant">load-ingredients</span>
  <span class="Comment"># if 'x' is null, return</span>
  <span class="muControl">return-unless</span> x
  next-node:&amp;:duplex-list:_elem<span class="Special"> &lt;- </span>get *x, <span class="Constant">next:offset</span>
  prev-node:&amp;:duplex-list:_elem<span class="Special"> &lt;- </span>get *x, <span class="Constant">prev:offset</span>
  <span class="Comment"># null x's pointers</span>
  *x<span class="Special"> &lt;- </span>put *x, <span class="Constant">next:offset</span>, <span class="Constant">0</span>
  *x<span class="Special"> &lt;- </span>put *x, <span class="Constant">prev:offset</span>, <span class="Constant">0</span>
  <span class="Comment"># if next-node is not null, set its prev pointer</span>
  <span class="Delimiter">{</span>
    <span class="muControl">break-unless</span> next-node
    *next-node<span class="Special"> &lt;- </span>put *next-node, <span class="Constant">prev:offset</span>, prev-node
  <span class="Delimiter">}</span>
  <span class="Comment"># if prev-node is not null, set its next pointer and return</span>
  <span class="Delimiter">{</span>
    <span class="muControl">break-unless</span> prev-node
    *prev-node<span class="Special"> &lt;- </span>put *prev-node, <span class="Constant">next:offset</span>, next-node
    <span class="muControl">return</span>
  <span class="Delimiter">}</span>
  <span class="Comment"># if prev-node is null, then we removed the head node at 'in'</span>
  <span class="Comment"># return the new head rather than the old 'in'</span>
  <span class="muControl">return</span> next-node
]

<span class="muScenario">scenario</span> removing-from-duplex-list [
  run [
    <span class="Constant">local-scope</span>
    list:&amp;:duplex-list:char<span class="Special"> &lt;- </span>push <span class="Constant">3</span>, <span class="Constant">0</span>
    list<span class="Special"> &lt;- </span>push <span class="Constant">4</span>, list
    list<span class="Special"> &lt;- </span>push <span class="Constant">5</span>, list
    list2:&amp;:duplex-list:char<span class="Special"> &lt;- </span>next list  <span class="Comment"># second element</span>
    list<span class="Special"> &lt;- </span>remove list2, list
    <span class="Constant">10</span>:bool/<span class="Special">raw &lt;- </span>equal list2, <span class="Constant">0</span>
    <span class="Comment"># check structure like before</span>
    list2<span class="Special"> &lt;- </span>copy list
    <span class="Constant">11</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>next list2
    <span class="Constant">12</span>:char/<span class="Special">raw &lt;- </span>first list2
    <span class="Constant">20</span>:&amp;:duplex-list:char/<span class="Special">raw &lt;- </span>next list2
    list2<span class="Special"> &lt;- </span>prev list2
    <span class="Constant">30</span>:char/<span class="Special">raw &lt;- </span>first list2
    <span class="Constant">40</span>:bool/<span class="Special">raw &lt;- </span>equal list, list2
  ]
  memory-should-contain [
    <span class="Constant">10</span><span class="Special"> &lt;- </span><span class="Constant">0</span>  <span class="Comment"># remove returned non-null</span>
    <span class="Constant">11</span><span class="Special"> &lt;- </span><span class="Constant">5</span>  <span class="Comment"># scanning next, skipping deleted element</span>
    <span class="Constant">12</span><span class="Special"> &lt;- </span><span class="Constant">3</span>
    <span class="Constant">20</span><span class="Special"> &lt;- </span><span class="Constant">0</span>  <span class="Comment"># no more elements</span>
    <span class="Constant">30</span><span class="Special"> &lt;- </span><span class="Constant">5</span>  <span class="Comment"># prev of final element</span>
    <span class="Constant">40</span><span class="Special"> &lt;- </span><span class="Constant">1</span>  <span class="Comment"># list back at start</span>
  ]
]

<span class="muScenario">scenario</span> removing-from-start-of-duplex-list [
  run [
    <span class="Constant">local-scope</span>
    list:&amp;:duplex-list:char<span class="Special"> &lt;- </span>push <span class="Constant">3</span>, <span class="Constant">0</span>
    list<span class="Special"> &lt;- </span>push <span class="Constant">4</span>, list
    list<span class="Special"> &lt;- </span>push <span class="Constant">5</span>, list
    list<span class="Special"> &lt;- </span>remove list, list
    <span class="Comment"># check structure like before</span>
    list2:&amp;:duplex-list:char<span class="Special"> &lt;- </span>copy list
    <span class="Constant">10</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>next list2
    <span class="Constant">11</span>:char/<span class="Special">raw &lt;- </span>first list2
    <span class="Constant">20</span>:&amp;:duplex-list:char/<span class="Special">raw &lt;- </span>next list2
    list2<span class="Special"> &lt;- </span>prev list2
    <span class="Constant">30</span>:char/<span class="Special">raw &lt;- </span>first list2
    <span class="Constant">40</span>:bool/<span class="Special">raw &lt;- </span>equal list, list2
  ]
  memory-should-contain [
    <span class="Constant">10</span><span class="Special"> &lt;- </span><span class="Constant">4</span>  <span class="Comment"># scanning next, skipping deleted element</span>
    <span class="Constant">11</span><span class="Special"> &lt;- </span><span class="Constant">3</span>
    <span class="Constant">20</span><span class="Special"> &lt;- </span><span class="Constant">0</span>  <span class="Comment"># no more elements</span>
    <span class="Constant">30</span><span class="Special"> &lt;- </span><span class="Constant">4</span>  <span class="Comment"># prev of final element</span>
    <span class="Constant">40</span><span class="Special"> &lt;- </span><span class="Constant">1</span>  <span class="Comment"># list back at start</span>
  ]
]

<span class="muScenario">scenario</span> removing-from-end-of-duplex-list [
  run [
    <span class="Constant">local-scope</span>
    list:&amp;:duplex-list:char<span class="Special"> &lt;- </span>push <span class="Constant">3</span>, <span class="Constant">0</span>
    list<span class="Special"> &lt;- </span>push <span class="Constant">4</span>, list
    list<span class="Special"> &lt;- </span>push <span class="Constant">5</span>, list
    <span class="Comment"># delete last element</span>
    list2:&amp;:duplex-list:char<span class="Special"> &lt;- </span>next list
    list2<span class="Special"> &lt;- </span>next list2
    list<span class="Special"> &lt;- </span>remove list2, list
    <span class="Constant">10</span>:bool/<span class="Special">raw &lt;- </span>equal list2, <span class="Constant">0</span>
    <span class="Comment"># check structure like before</span>
    list2<span class="Special"> &lt;- </span>copy list
    <span class="Constant">11</span>:char/<span class="Special">raw &lt;- </span>first list2
    list2<span class="Special"> &lt;- </span>next list2
    <span class="Constant">12</span>:char/<span class="Special">raw &lt;- </span>first list2
    <span class="Constant">20</span>:&amp;:duplex-list:char/<span class="Special">raw &lt;- </span>next list2
    list2<span class="Special"> &lt;- </span>prev list2
    <span class="Constant">30</span>:char/<span class="Special">raw &lt;- </span>first list2
    <span class="Constant">40</span>:bool/<span class="Special">raw &lt;- </span>equal list, list2
  ]
  memory-should-contain [
    <span class="Constant">10</span><span class="Special"> &lt;- </span><span class="Constant">0</span>  <span class="Comment"># remove returned non-null</span>
    <span class="Constant">11</span><span class="Special"> &lt;- </span><span class="Constant">5</span>  <span class="Comment"># scanning next, skipping deleted element</span>
    <span class="Constant">12</span><span class="Special"> &lt;- </span><span class="Constant">4</span>
    <span class="Constant">20</span><span class="Special"> &lt;- </span><span class="Constant">0</span>  <span class="Comment"># no more elements</span>
    <span class="Constant">30</span><span class="Special"> &lt;- </span><span class="Constant">5</span>  <span class="Comment"># prev of final element</span>
    <span class="Constant">40</span><span class="Special"> &lt;- </span><span class="Constant">1</span>  <span class="Comment"># list back at start</span>
  ]
]

<span class="muScenario">scenario</span> removing-from-singleton-duplex-list [
  run [
    <span class="Constant">local-scope</span>
    list:&amp;:duplex-list:char<span class="Special"> &lt;- </span>push <span class="Constant">3</span>, <span class="Constant">0</span>
    list<span class="Special"> &lt;- </span>remove list, list
    <span class="Constant">1</span>:num/<span class="Special">raw &lt;- </span>copy list
  ]
  memory-should-contain [
    <span class="Constant">1</span><span class="Special"> &lt;- </span><span class="Constant">0</span>  <span class="Comment"># back to an empty list</span>
  ]
]

<span class="Comment"># remove values between 'start' and 'end' (both exclusive).</span>
<span class="Comment"># also clear pointers back out from start/end for hygiene.</span>
<span class="Comment"># set end to 0 to delete everything past start.</span>
<span class="Comment"># can't set start to 0 to delete everything before end, because there's no</span>
<span class="Comment"># clean way to return the new head pointer.</span>
<span class="muRecipe">def</span> remove-between start:&amp;:duplex-list:_elem, end:&amp;:duplex-list:_elem/contained-in:start<span class="muRecipe"> -&gt; </span>start:&amp;:duplex-list:_elem [
  <span class="Constant">local-scope</span>
  <span class="Constant">load-ingredients</span>
  next:&amp;:duplex-list:_elem<span class="Special"> &lt;- </span>get *start, <span class="Constant">next:offset</span>
  nothing-to-delete?:bool<span class="Special"> &lt;- </span>equal next, end
  <span class="muControl">return-if</span> nothing-to-delete?
  assert next, <span class="Constant">[malformed duplex list]</span>
  <span class="Comment"># start-&gt;next-&gt;prev = 0</span>
  <span class="Comment"># start-&gt;next = end</span>
  *next<span class="Special"> &lt;- </span>put *next, <span class="Constant">prev:offset</span>, <span class="Constant">0</span>
  *start<span class="Special"> &lt;- </span>put *start, <span class="Constant">next:offset</span>, end
  <span class="muControl">return-unless</span> end
  <span class="Comment"># end-&gt;prev-&gt;next = 0</span>
  <span class="Comment"># end-&gt;prev = start</span>
  prev:&amp;:duplex-list:_elem<span class="Special"> &lt;- </span>get *end, <span class="Constant">prev:offset</span>
  assert prev, <span class="Constant">[malformed duplex list - 2]</span>
  *prev<span class="Special"> &lt;- </span>put *prev, <span class="Constant">next:offset</span>, <span class="Constant">0</span>
  *end<span class="Special"> &lt;- </span>put *end, <span class="Constant">prev:offset</span>, start
]

<span class="muScenario">scenario</span> remove-range [
  <span class="Comment"># construct a duplex list with six elements [13, 14, 15, 16, 17, 18]</span>
  <span class="Constant">local-scope</span>
  list:&amp;:duplex-list:char<span class="Special"> &lt;- </span>push <span class="Constant">18</span>, <span class="Constant">0</span>
  list<span class="Special"> &lt;- </span>push <span class="Constant">17</span>, list
  list<span class="Special"> &lt;- </span>push <span class="Constant">16</span>, list
  list<span class="Special"> &lt;- </span>push <span class="Constant">15</span>, list
  list<span class="Special"> &lt;- </span>push <span class="Constant">14</span>, list
  list<span class="Special"> &lt;- </span>push <span class="Constant">13</span>, list
  <span class="Constant">1</span>:&amp;:duplex-list:char/<span class="Special">raw &lt;- </span>copy list  <span class="Comment"># save list</span>
  run [
    <span class="Constant">local-scope</span>
    list:&amp;:duplex-list:char<span class="Special"> &lt;- </span>copy <span class="Constant">1</span>:&amp;:duplex-list:char/<span class="Special">raw</span>  <span class="Comment"># restore list</span>
    <span class="Comment"># delete 16 onwards</span>
    <span class="Comment"># first pointer: to the third element</span>
    list2:&amp;:duplex-list:char<span class="Special"> &lt;- </span>next list
    list2<span class="Special"> &lt;- </span>next list2
    list2<span class="Special"> &lt;- </span>remove-between list2, <span class="Constant">0</span>
    <span class="Comment"># now check the list</span>
    <span class="Constant">10</span>:char/<span class="Special">raw &lt;- </span>get *list, <span class="Constant">value:offset</span>
    list<span class="Special"> &lt;- </span>next list
    <span class="Constant">11</span>:char/<span class="Special">raw &lt;- </span>get *list, <span class="Constant">value:offset</span>
    list<span class="Special"> &lt;- </span>next list
    <span class="Constant">12</span>:char/<span class="Special">raw &lt;- </span>get *list, <span class="Constant">value:offset</span>
    <span class="Constant">20</span>:&amp;:duplex-list:char/<span class="Special">raw &lt;- </span>next list
  ]
  memory-should-contain [
    <span class="Constant">10</span><span class="Special"> &lt;- </span><span class="Constant">13</span>
    <span class="Constant">11</span><span class="Special"> &lt;- </span><span class="Constant">14</span>
    <span class="Constant">12</span><span class="Special"> &lt;- </span><span class="Constant">15</span>
    <span class="Constant">20</span><span class="Special"> &lt;- </span><span class="Constant">0</span>
  ]
]

<span class="muScenario">scenario</span> remove-range-to-final [
  <span class="Constant">local-scope</span>
  <span class="Comment"># construct a duplex list with six elements [13, 14, 15, 16, 17, 18]</span>
  list:&amp;:duplex-list:char<span class="Special"> &lt;- </span>push <span class="Constant">18</span>, <span class="Constant">0</span>
  list<span class="Special"> &lt;- </span>push <span class="Constant">17</span>, list
  list<span class="Special"> &lt;- </span>push <span class="Constant">16</span>, list
  list<span class="Special"> &lt;- </span>push <span class="Constant">15</span>, list
  list<span class="Special"> &lt;- </span>push <span class="Constant">14</span>, list
  list<span class="Special"> &lt;- </span>push <span class="Constant">13</span>, list
  <span class="Constant">1</span>:&amp;:duplex-list:char/<span class="Special">raw &lt;- </span>copy list  <span class="Comment"># save list</span>
  run [
    <span class="Constant">local-scope</span>
    list:&amp;:duplex-list:char<span class="Special"> &lt;- </span>copy <span class="Constant">1</span>:&amp;:duplex-list:char/<span class="Special">raw</span>  <span class="Comment"># restore list</span>
    <span class="Comment"># delete 15, 16 and 17</span>
    <span class="Comment"># start pointer: to the second element</span>
    list2:&amp;:duplex-list:char<span class="Special"> &lt;- </span>next list
    <span class="Comment"># end pointer: to the last (sixth) element</span>
    end:&amp;:duplex-list:char<span class="Special"> &lt;- </span>next list2
    end<span class="Special"> &lt;- </span>next end
    end<span class="Special"> &lt;- </span>next end
    end<span class="Special"> &lt;- </span>next end
    remove-between list2, end
    <span class="Comment"># now check the list</span>
    <span class="Constant">10</span>:char/<span class="Special">raw &lt;- </span>get *list, <span class="Constant">value:offset</span>
    list<span class="Special"> &lt;- </span>next list
    <span class="Constant">11</span>:char/<span class="Special">raw &lt;- </span>get *list, <span class="Constant">value:offset</span>
    list<span class="Special"> &lt;- </span>next list
    <span class="Constant">12</span>:char/<span class="Special">raw &lt;- </span>get *list, <span class="Constant">value:offset</span>
    <span class="Constant">20</span>:&amp;:duplex-list:char/<span class="Special">raw &lt;- </span>next list
  ]
  memory-should-contain [
    <span class="Constant">10</span><span class="Special"> &lt;- </span><span class="Constant">13</span>
    <span class="Constant">11</span><span class="Special"> &lt;- </span><span class="Constant">14</span>
    <span class="Constant">12</span><span class="Special"> &lt;- </span><span class="Constant">18</span>
    <span class="Constant">20</span><span class="Special"> &lt;- </span><span class="Constant">0</span>  <span class="Comment"># no more elements</span>
  ]
]

<span class="muScenario">scenario</span> remove-range-empty [
  <span class="Constant">local-scope</span>
  <span class="Comment"># construct a duplex list with three elements [13, 14, 15]</span>
  list:&amp;:duplex-list:char<span class="Special"> &lt;- </span>push <span class="Constant">15</span>, <span class="Constant">0</span>
  list<span class="Special"> &lt;- </span>push <span class="Constant">14</span>, list
  list<span class="Special"> &lt;- </span>push <span class="Constant">13</span>, list
  <span class="Constant">1</span>:&amp;:duplex-list:char/<span class="Special">raw &lt;- </span>copy list  <span class="Comment"># save list</span>
  run [
    <span class="Constant">local-scope</span>
    list:&amp;:duplex-list:char<span class="Special"> &lt;- </span>copy <span class="Constant">1</span>:&amp;:duplex-list:char/<span class="Special">raw</span>  <span class="Comment"># restore list</span>
    <span class="Comment"># delete between first and second element (i.e. nothing)</span>
    list2:&amp;:duplex-list:char<span class="Special"> &lt;- </span>next list
    remove-between list, list2
    <span class="Comment"># now check the list</span>
    <span class="Constant">10</span>:char/<span class="Special">raw &lt;- </span>get *list, <span class="Constant">value:offset</span>
    list<span class="Special"> &lt;- </span>next list
    <span class="Constant">11</span>:char/<span class="Special">raw &lt;- </span>get *list, <span class="Constant">value:offset</span>
    list<span class="Special"> &lt;- </span>next list
    <span class="Constant">12</span>:char/<span class="Special">raw &lt;- </span>get *list, <span class="Constant">value:offset</span>
    <span class="Constant">20</span>:&amp;:duplex-list:char/<span class="Special">raw &lt;- </span>next list
  ]
  <span class="Comment"># no change</span>
  memory-should-contain [
    <span class="Constant">10</span><span class="Special"> &lt;- </span><span class="Constant">13</span>
    <span class="Constant">11</span><span class="Special"> &lt;- </span><span class="Constant">14</span>
    <span class="Constant">12</span><span class="Special"> &lt;- </span><span class="Constant">15</span>
    <span class="Constant">20</span><span class="Special"> &lt;- </span><span class="Constant">0</span>
  ]
]

<span class="muScenario">scenario</span> remove-range-to-end [
  <span class="Constant">local-scope</span>
  <span class="Comment"># construct a duplex list with six elements [13, 14, 15, 16, 17, 18]</span>
  list:&amp;:duplex-list:char<span class="Special"> &lt;- </span>push <span class="Constant">18</span>, <span class="Constant">0</span>
  list<span class="Special"> &lt;- </span>push <span class="Constant">17</span>, list
  list<span class="Special"> &lt;- </span>push <span class="Constant">16</span>, list
  list<span class="Special"> &lt;- </span>push <span class="Constant">15</span>, list
  list<span class="Special"> &lt;- </span>push <span class="Constant">14</span>, list
  list<span class="Special"> &lt;- </span>push <span class="Constant">13</span>, list
  <span class="Constant">1</span>:&amp;:duplex-list:char/<span class="Special">raw &lt;- </span>copy list  <span class="Comment"># save list</span>
  run [
    <span class="Constant">local-scope</span>
    list:&amp;:duplex-list:char<span class="Special"> &lt;- </span>copy <span class="Constant">1</span>:&amp;:duplex-list:char/<span class="Special">raw</span>  <span class="Comment"># restore list</span>
    <span class="Comment"># remove the third element and beyond</span>
    list2:&amp;:duplex-list:char<span class="Special"> &lt;- </span>next list
    remove-between list2, <span class="Constant">0</span>
    <span class="Comment"># now check the list</span>
    <span class="Constant">10</span>:char/<span class="Special">raw &lt;- </span>get *list, <span class="Constant">value:offset</span>
    list<span class="Special"> &lt;- </span>next list
    <span class="Constant">11</span>:char/<span class="Special">raw &lt;- </span>get *list, <span class="Constant">value:offset</span>
    <span class="Constant">20</span>:&amp;:duplex-list:char/<span class="Special">raw &lt;- </span>next list
  ]
  memory-should-contain [
    <span class="Constant">10</span><span class="Special"> &lt;- </span><span class="Constant">13</span>
    <span class="Constant">11</span><span class="Special"> &lt;- </span><span class="Constant">14</span>
    <span class="Constant">20</span><span class="Special"> &lt;- </span><span class="Constant">0</span>
  ]
]

<span class="Comment"># insert list beginning at 'new' after 'in'</span>
<span class="muRecipe">def</span> insert-range in:&amp;:duplex-list:_elem, start:&amp;:duplex-list:_elem/contained-in:in<span class="muRecipe"> -&gt; </span>in:&amp;:duplex-list:_elem [
  <span class="Constant">local-scope</span>
  <span class="Constant">load-ingredients</span>
  <span class="muControl">return-unless</span> in
  <span class="muControl">return-unless</span> start
  end:&amp;:duplex-list:_elem<span class="Special"> &lt;- </span>copy start
  <span class="Delimiter">{</span>
    next:&amp;:duplex-list:_elem<span class="Special"> &lt;- </span>next end/insert-range
    <span class="muControl">break-unless</span> next
    end<span class="Special"> &lt;- </span>copy next
    <span class="muControl">loop</span>
  <span class="Delimiter">}</span>
  next:&amp;:duplex-list:_elem<span class="Special"> &lt;- </span>next in
  *end<span class="Special"> &lt;- </span>put *end, <span class="Constant">next:offset</span>, next
  <span class="Delimiter">{</span>
    <span class="muControl">break-unless</span> next
    *next<span class="Special"> &lt;- </span>put *next, <span class="Constant">prev:offset</span>, end
  <span class="Delimiter">}</span>
  *in<span class="Special"> &lt;- </span>put *in, <span class="Constant">next:offset</span>, start
  *start<span class="Special"> &lt;- </span>put *start, <span class="Constant">prev:offset</span>, in
]

<span class="muRecipe">def</span> append in:&amp;:duplex-list:_elem, new:&amp;:duplex-list:_elem/contained-in:in<span class="muRecipe"> -&gt; </span>in:&amp;:duplex-list:_elem [
  <span class="Constant">local-scope</span>
  <span class="Constant">load-ingredients</span>
  last:&amp;:duplex-list:_elem<span class="Special"> &lt;- </span>last in
  *last<span class="Special"> &lt;- </span>put *last, <span class="Constant">next:offset</span>, new
  <span class="muControl">return-unless</span> new
  *new<span class="Special"> &lt;- </span>put *new, <span class="Constant">prev:offset</span>, last
]

<span class="muRecipe">def</span> last in:&amp;:duplex-list:_elem<span class="muRecipe"> -&gt; </span>result:&amp;:duplex-list:_elem [
  <span class="Constant">local-scope</span>
  <span class="Constant">load-ingredients</span>
  result<span class="Special"> &lt;- </span>copy in
  <span class="Delimiter">{</span>
    next:&amp;:duplex-list:_elem<span class="Special"> &lt;- </span>next result
    <span class="muControl">break-unless</span> next
    result<span class="Special"> &lt;- </span>copy next
    <span class="muControl">loop</span>
  <span class="Delimiter">}</span>
]

<span class="Comment"># helper for debugging</span>
<span class="muRecipe">def</span> dump-from x:&amp;:duplex-list:_elem [
  <span class="Constant">local-scope</span>
  <span class="Constant">load-ingredients</span>
  $print x, <span class="Constant">[: ]</span>
  <span class="Delimiter">{</span>
    <span class="muControl">break-unless</span> x
    c:_elem<span class="Special"> &lt;- </span>get *x, <span class="Constant">value:offset</span>
    $print c, <span class="Constant">[ ]</span>
    x<span class="Special"> &lt;- </span>next x
    <span class="Delimiter">{</span>
      is-newline?:bool<span class="Special"> &lt;- </span>equal c, <span class="Constant">10/newline</span>
      <span class="muControl">break-unless</span> is-newline?
      $print <span class="Constant">10/newline</span>
      $print x, <span class="Constant">[: ]</span>
    <span class="Delimiter">}</span>
    <span class="muControl">loop</span>
  <span class="Delimiter">}</span>
  $print <span class="Constant">10/newline</span>, <span class="Constant">[---]</span>, <span class="Constant">10/newline</span>
]
</pre>
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