3986 - bring back delimited continuations

They're back after a long hiatus: commit 2295 in Oct 2015.

I'm not convinced anymore that this is actually a correct implementation
of continuations. Issues on at least two fronts:

a) These aren't safe yet. Since continuations can be called multiple
times, we need to disable reclamation of locals inside a continuation.
There may be other type- or memory-safety issues. However, delimited
continuations at least seem possible to make safe. Undelimited
continuations (call/cc) though are permanently out.

b) They may not actually be as powerful as delimited continuations.
Let's see if I can build 'yield' out of these primitives.

1 files changed, 4 insertions, 1 deletions

diff --git a/071recipe.cc b/071recipe.cc
index a3a258c7..92ac83ac 100644
--- a/071recipe.cc
+++ b/071recipe.cc
@@ -208,6 +208,7 @@ recipe from_reagent(const reagent& r) {
   assert(r.type);
   recipe result_header;  // will contain only ingredients and products, nothing else
   result_header.has_header = true;
+  // Begin Reagent->Recipe(r, recipe_header)
   if (r.type->atom) {
     assert(r.type->name == "recipe");
     return result_header;
@@ -275,8 +276,10 @@ reagent next_recipe_reagent(const type_tree* curr) {
 
 bool is_mu_recipe(const reagent& r) {
   if (!r.type) return false;
-  if (r.type->atom)
+  if (r.type->atom) {
+    // End is_mu_recipe Atom Cases(r)
     return r.type->name == "recipe-literal";
+  }
   return r.type->left->atom && r.type->left->name == "recipe";
 }