Module type Container_intf.Make_arg

module type Make_arg = sig .. end

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