Set: pp::colors
  Set: pp::unicode
  Assumed: a
  Assumed: b
  Assumed: c
  Assumed: d
  Assumed: H
  Set: lean::pp::implicit
  Using: Nat
Nat::add_succr a : ∀ b : ℕ, @eq ℕ (a + (b + 1)) (a + b + 1)
  Proved: mul_zerol2