• Expression Stack at time t = [ [ ], [ [ obj.append(x), ys ], rest ] ]
• Value Stack at time t = [ [ value, [ ] ], [ next_level, other_levels ] ]
• value at obj 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 Value Stack at time (t + 1) = [ [ None, next_level ], other_levels ]