Goal:
    A : Type  B : Type  R : A → B → Type  x : A  y : list B
  ============================
   (All2 R [x] y → ∑ y' : B, y = [y'] × R x y')


DEBUG:Going to execute:
intros a
DEBUG LTAC Evaluated term: All2_map_equiv
DEBUG LTAC Evaluated term: All2_map_equiv