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 ### Data Structures
 
+For memory safety, the following data structures are opaque and only modified
+using functions described further down. I still find it useful to understand
+how they work under the hood.
+
 - Handles: addresses to objects allocated on the heap. They're augmented with
   book-keeping to guarantee memory-safety, and so cannot be stored in registers.
   See [mu.md](mu.md) for details, but in brief: