Set: pp::colors
  Set: pp::unicode
  Using: Nat
0 + 1
Nat::add_assoc : ∀ a b c : ℕ, a + (b + c) = a + b + c
using.lean:7:6: error: unknown identifier 'add'
  Using: Nat
0 + 1
  Using: Nat::add as plus
using.lean:15:6: error: unknown identifier 'plus_assoc'
using.lean:17:6: error: unknown identifier 'add'