polymorphic recursion clarification

["Jacques Le Normand" <rathereasy@gmail.com>]

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maybe I should state why I need polymorphic recursion
consider the function traverse which takes a function and an element of what
I want to traverse and returns a list of results.

let rec traverse f n=
  let ret = f n in
  let rest = match n#get_right_sibling_specific with
      None -> []
    | Some y -> traverse f y
  in
  let rest2 = match n#get_child_specific with
      None -> []
    | Some y -> traverse f y
  in
    match ret with
    None -> rest @ rest2
      | Some y -> y :: rest @ rest2


with type

val traverse :
  ((< get_child_specific : 'a option; get_right_sibling_specific : 'a
option;
      .. >
    as 'a) ->
   'b option) ->
  'a -> 'b list

the problem is that get_child_specific may not return an 'a option, but it
will always return a subtype of 'a option. is there any way to do this? ie,
give it a type

((< get_child_specific : #'a option; get_right_sibling_specific : #'a
option;
      .. >
    as 'a) ->
   'b option) ->
  'a -> 'b list

? (I believe this can be done with polymorphic recursion)
--Jacques

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