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


DEBUG:Going to execute:
try intros until H; intro_block_id H || intro_block H;
            (try simpl in H; DepElim.simplify_equations_in H)