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,
       (exists e : elt, find k m = Some e) <->
       (exists e : elt, find k m' = Some e)
  H3 : forall (k : key) (e e' : elt),
       find k m = Some e -> find k m' = Some e' -> eq_elt e e'
  k : key
  H2 : exists e : elt, find k m = Some e
  H4 : forall k : key,
       (exists e : elt, find k m = Some e) ->
       exists e : elt, find k m' = Some e
  ============================
   (forall k : key,
    (exists e : elt, find k m' = Some e) -> exists e : elt, find k m = Some e)


DEBUG:Going to execute:
tac H
DEBUG LTAC Evaluated term: (forall k : key,
                            (exists e : elt, find k m' = Some e) ->
                            exists e : elt, find k m = Some e)