Working with ocaml's functional data structures has quickly become a job of
connecting different functors together, and I'd appreciate some help.

I have the following layout:

module rec OrderedKey : OrderedKeyType = struct
    type t = Value.value_t
    let compare = compare
    let filter_idxs idxs = function
      | Value.VTuple l -> Value.VTuple(list_filter_idxs idxs l)
      | _ -> invalid_arg "not a vtuple"
    end

and ValueBag : IBag.S with type elt = Value.value_t =
  IBag.Make(OrderedKey)

and ValueMMap : IMultimap.S with type elt = Value.value_t and type bag =
ValueBag.t =
  IMultimap.Make(OrderedKey)

and Value : sig ... type value_t = ... end = Value

The situation is as such: my multimap (IMultimap) contains an internal
specialization of the IBag functor called an InnerBag. It attempts to
return said bag, which is equivalent to the external ValueBag in structure.
However, I don't know how to tell ocaml that the type 'bag' which is
abstract in IMultimap is exactly the same as the external ValueBag. I tried
to do that above, but what I get is a mismatch between IMultimap's internal
InnerBag.t and the external ValueBag.t.

Any help would be appreciated.

Yotam