if the following are true:

• Expression Stack at time t = [ [ ], [ [ function call with name: "__setitem__" and arguments: args, ys ], rest ] ]
• Value Stack at time t = [ [ value, [ key, [ Python reference idx, [ ] ] ] ], [ next_level, other_levels ] ]
• the element at index idx of stack (Python Object Store at time t) = Python list elements

then Value Stack at time (t + 1) = [ [ None, next_level ], other_levels ]