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
  x : elt
  H3 : forall e : elt, find k m = Some e -> eq_elt e x
  H1 : find k m' = Some x
  H4 : forall (k : key) (e : elt),
       find k m = Some e -> exists e0 : elt, find k m' = Some e0
  H5 : exists e : elt, find k m = Some e
  H2 : forall k : key, find k m' = find k m'
  H6 : forall k : key, find k m = find k m
  ============================
   (find k m = Some ?e)


DEBUG:Going to execute:
tac