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:
fun tac H =>
             DepElim.do_intros H; DepElim.generalize_by_eqs H; tac H