• Value Stack at time t = [ [ a, [ ] ], [ next_level, other_levels ] ]
• Expression Stack at time t = [ [ ], [ [ name."pop"(args), ys ], rest ] ]
• value at name in map (Variables Map at time t) = Python reference idx
• the element at index idx of stack (Python Object Store at time t) = Python list entries

then Python Object Store at time (t + 1) = result of storing (Python list (stack after popping a value from stack entries at index a)) at index idx of stack (Python Object Store at time t)