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:
DepElim.do_intros H; DepElim.generalize_by_eqs H
Level 1: evaluation returns
DepElim.do_intros H; DepElim.generalize_by_eqs H; tac H
where
tac := fun hyp => DepElim.do_case hyp
       where
       id := a
       of type
       ident
of type
tacvalue
H := a
of type
ident of type tacvalue