if the following are true:

• the line at time 22 = 3
• the tab at time 22 = 1
• statement at line 3, tab 1 = a = __add__(a, 1)
• expression state at time 22 = "end_expr"

then the tab at time 23 = 1

Start from the conclusion and work back up the proof. Click the arrow to show the parents.