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
• Context Stack at time t = [ program context with variables: vars and expression state: (expression state with expression stack: expr_stack values: value_stack line: line tab: tab) and control map: control_map, rest ]
• Expression Stack at time t = [ [ ], [ ] ]

then Expression Stack at time (t + 1) = expr_stack