Replace custom stack implementation with collections.ChainMap
Implement decorator to eliminate unused lines of code (assignments to unused values)
Technically, x += y doesn't have to be the same thing as x = x + y. Handle it as its own operation of the form x += y; return x
Support efficiently inlining simple functions, i.e. where there is no return or only one return as the last line of the function, using pure name substitution without loops, try/except, or anything else fancy
Catch replacement of loop variables that conflict with globals, or throw a more descriptive error when detected. See test_iteration_variable