if the following are true:

• the line at time t = i
• the tab at time t = j
• statement at line i, tab j = return rex
• stack at time t = [ program context with variables: varis and expression state: (expression state with parent stack: parent_stack arguments: arg_stack values: value_stack line: line tab: tab) and control map: control_map, rest ]
• expression state at time t = "end_expr"

then parent stack at time (t + 1) = parent_stack