Goal:
  
  elt : Type
  eq_elt : elt -> elt -> Prop
  m : t elt
  m' : t elt
  H : forall x y : key, E.eq x y -> find x m = find y m
  H0 : forall x y : key, E.eq x y -> find x m' = find y m'
  H1 : forall (k : key) (e : elt),
       find k m = Some e ->
       match find k m with
       | Some x =>
           match find k m' with
           | Some y => eq_elt x y
           | None => False
           end
       | None => match find k m' with
                 | Some _ => False
                 | None => True
                 end
       end
  H3 : forall (k : key) (e : elt),
       find k m' = Some e ->
       match find k m with
       | Some x =>
           match find k m' with
           | Some y => eq_elt x y
           | None => False
           end
       | None => match find k m' with
                 | Some _ => False
                 | None => True
                 end
       end
  k : key
  x : elt
  H2 : find k m' = Some x
  ============================
   (exists e : elt, find k m = Some e)


DEBUG:Going to execute:
idtac
Level 2: evaluation returns
idtac;
 (let is_transparent :=
   match goal with
   | _ => let __ := eval cbv delta [H] in H in
          constr:(true)
   | _ => constr:(false)
   end
  in
  lazymatch constr:((all_ways, is_transparent)) with
  | (false, true) =>
      let H' := fresh in
      rename H into H';
       (let H'' := guarded_specialize_term_under_binders_by guard_tac H' tac
        in
        pose (H := H''); subst H')
  | (true, true) =>
      let H := guarded_specialize_term_under_binders_by guard_tac H tac in
      unique
      pose
      H
  | (false, false) =>
      let H := guarded_specialize_term_under_binders_by guard_tac H tac in
      specialize
      H
  | (true, false) =>
      let H := guarded_specialize_term_under_binders_by guard_tac H tac in
      unique
      pose
      proof
      H
  end)
where
tac := apply E.eq_refl
of type
tacvalue
guard_tac := fun H => lazymatch type of H with
                      | ?A -> ?B => idtac
                      end
of type
tacvalue
all_ways := true
of type
constr
H := x
of type
constr of type tacvalue