1 subgoal
  
  y1 := 0 : nat
  x := 0 + 0 : nat
  ============================
   x = x
1 subgoal
  
  y1 := 0 : nat
  y2 := 0 : nat
  x := y2 + 0 : nat
  ============================
   x = x
1 subgoal
  
  y1 := 0 : nat
  y2 := 0 : nat
  y3 := 0 : nat
  x := y2 + y3 : nat
  ============================
   x = x