forall m n : nat, m * n = (2 * m * n)%nat
     : Prop
File "stdin", line 11, characters 0-31:
Warning: Notation "_ * _" was already used in scope nat_scope.
[notation-overridden,parsing]
forall m n : nat, m * n = Nat.mul (Nat.mul 2 m) n
     : Prop