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'
  k : key
  e : elt
  E : find k m = Some e
  E0 : find k m' = None
  H1 : False
  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
  x : elt
  H2 : Some e = Some x
  H4 : forall k : key, find k m' = find k m'
  H5 : forall k : key, find k m = find k m
  ============================
   (exists e : elt, None = Some e)


DEBUG:Going to execute:
match reverse goal with
           | id:?X1
             |- _ =>
                 <Coq.Init.Tauto.is_conj> flags X1; elim id; do 2 intro;
                  clear id
           | id:_ <-> _ |- _ => elim id; do 2 intro; clear id
           | id:~ _ |- _ => red in id
           | id:?X1
             |- _ =>
                 <Coq.Init.Tauto.is_disj> flags X1; elim id; intro; clear id
           | _ =>
               <Coq.Init.Tauto.find_cut>
                ltac:(fun id0 id1 X2 =>
                        assert X2; [ exact (id0 id1) | clear id0 ])
           | id:?X1 -> ?X2
             |- _ =>
                 <Coq.Init.Tauto.is_unit_or_eq> flags X1; cut X2;
                  [ intro; clear id
                  | cut X1; [ exact id | constructor 1; fail ] ]
           | id:?X1 -> ?X2
             |- _ =>
                 <Coq.Init.Tauto.flatten_contravariant_conj> flags X1 X2 id
           | id:?X1 <-> ?X2 -> ?X3
             |- _ =>
                 assert ((X1 -> X2) -> (X2 -> X1) -> X3) by
                  (do 2 intro; apply id; split; assumption); clear id
           | id:?X1 -> ?X2
             |- _ =>
                 <Coq.Init.Tauto.flatten_contravariant_disj> flags X1 X2 id
           | |- ?X1 => <Coq.Init.Tauto.is_conj> flags X1; split
           | |- _ <-> _ => split
           | |- ~ _ => red
           end; <Coq.Init.Tauto.not_dep_intros>
Level 2: evaluation returns
<coq-core.plugins.tauto::is_conj@0>
where
tauto_flags := 
of type
tauto_flags
X1 := exists e : elt, Some e0 = Some e
of type
constr of type tacvalue

Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.
Level 0: Backtrace:

No matching clauses for match.
Level 0: Backtrace:

No matching clauses for match.