Module type Make_gen_arg


type 'a t
type 'a elt
val fold : 'a t -> init:'accum -> f:('accum -> 'a elt -> 'accum) -> 'accum
val iter : [
| `Define_using_fold
| `Custom of 'a t -> f:('a elt -> unit) -> unit

The iter argument to Container.Make says how to implement the container's iter function. `Define_using_fold means to define iter via:

        iter t ~f = Container.iter ~fold t ~f

`Custom overrides the default implementation, presumably with something more efficient. Several other functions returned by Container.Make are defined in terms of iter, so passing in a more efficient iter will improve their efficiency as well.