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'
  ============================
   ((forall k : key,
     (exists e : elt, MapsTo k e m) ->
     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) /\
    (forall k : key,
     (exists e : elt, MapsTo k e m') ->
     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) <->
    (forall k : key,
     (exists e : elt, find k m = Some e) <->
     (exists e : elt, find k m' = Some e)) /\
    (forall (k : key) (e e' : elt),
     find k m = Some e -> find k m' = Some e' -> eq_elt e e'))


DEBUG:Going to execute:
setoid_rewrite find_mapsto_iff