Theorem: Pos.mul_succ (Extra/Pos/Lemmas.lean)

State before:
x y : Pos
⊢ x * (y + 1) = x * y + x
Tactic:
by_toNat
State after:
x y : Pos
⊢ x.toNat * (y.toNat + 1) = x.toNat * y.toNat + x.toNat

State before:
x y : Pos
⊢ x.toNat * (y.toNat + 1) = x.toNat * y.toNat + x.toNat
Tactic:
exact Nat.mul_succ ..
State after:
no goals