> modifying a collection from inside foreach loop This is exactly what the mutable map pointer was for, for the function to be able to modify the collection; C

> modifying a collection from inside foreach loop This is exactly what the mutable map pointer was for, for the function to be able to modify the collection; C++ would result in potentially iterating garbage, C# it sounds like would throw an exception (and so show the design wouldn't work when I tried to test it), Python definitely didn't do a graceful thing when I tried it just now. And if I had a collection struct in C, I'm sure I could've done some horrible things with it when trying. The best of those outcomes is C#, which would've shown me my design was bad when I ran it; that could be as early as the code was written if tests are written at every step. But it could be quite a bit later, after several other interacting parts get written, and you rely on what turns out to be an invalid assumption. For Rust, the compiler alerted me to the problem the moment I added the code. FTR I ended up accumulating changes to the map during the loop, and only applying them after the loop had finished. EDIT: Python did do something sensible: I didn't expect pop'ing an element from the list to echo the element popped in the REPL, and got a printed interleaved from front to back, which does makes sense.