Set: pp::colors
  Set: pp::unicode
  Assumed: C
  Assumed: D
  Assumed: R
  Proved: R2
Theorem R2 (A1 A2 B1 B2 : Type) (H : A1 → B1 = A2 → B2) (a : A1) : B1 = B2 :=
    R A1 A2 (λ x : A1, B1) (λ x : A2, B2) H a