Set: pp::colors
  Set: pp::unicode
  Defined: xor
⊤ ⊕ ⊥
⊥
⊤
  Assumed: a
a ⊕ a ⊕ a
@subst : Π (A : TypeU) (a b : A) (P : A → Bool), P a → a == b → P b
  Proved: EM2
EM2 : Π a : Bool, a ∨ ¬ a
EM2 a : a ∨ ¬ a