Set: pp::colors
  Set: pp::unicode
  Assumed: f
  Proved: T1
  Proved: T2
Theorem T1 (a b c : ℤ) (H1 : a = b) (H2 : a = c) : (f (f a a) b) = (f (f b c) a) :=
    Congr (Congr (Refl f) (Congr (Congr (Refl f) H1) H2)) (Symm H1)
Theorem T2 (a b c : ℤ) (H1 : a = b) (H2 : a = c) : (f (f a c)) = (f (f b a)) :=
    Congr (Refl f) (Congr (Congr (Refl f) H1) (Symm H2))