theorem insert_inj: \forall S1,i,P, Q. Insert S1 i P Q \to
\forall S2. Insert S2 i P Q \to S1 = S2.
intros 5; elim H; clear H i P Q;
theorem insert_inj: \forall S1,i,P, Q. Insert S1 i P Q \to
\forall S2. Insert S2 i P Q \to S1 = S2.
intros 5; elim H; clear H i P Q;