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

then Value Stack at time (t + 1) = [ [ stack entries contains a, next_level ], other_levels ]