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