Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  x : A
  y0 : B
  l' : list B
  r : R x y0
  a : All2 R [] l'
  ============================
   (∑ y' : B, y0 :: l' = [y'] × R x y')


DEBUG:Going to execute:
DepElim.do_depelim_nosimpl tac H; DepElim.simpl_dep_elim
Level 3: evaluation returns
DepElim.do_depelim_nosimpl tac H; DepElim.simpl_dep_elim;
 CoreTactics.unblock_goal
where
tac := fun hyp => DepElim.do_case hyp
       where
       id := a
       of type
       constr
of type
tacvalue
H := a
of type
constr of type tacvalue

Level 2: evaluation returns
DepElim.do_depelim_nosimpl tac H; DepElim.simpl_dep_elim;
 CoreTactics.unblock_goal
where
tac := fun hyp => DepElim.do_case hyp
       where
       id := a
       of type
       constr
of type
tacvalue
H := a
of type
constr of type tacvalue

Level 1: evaluation returns
DepElim.do_depelim_nosimpl tac H; DepElim.simpl_dep_elim;
 CoreTactics.unblock_goal
where
tac := fun hyp => DepElim.do_case hyp
       where
       id := a
       of type
       constr
of type
tacvalue
H := a
of type
constr of type tacvalue