Another solution is to use polymorphic variants [1] :

type 'ud type_tree = [ `Primitive of 'ud * string |
                      `Pointer of 'ud * 'ud type_tree ]
type 'ud expr_tree = [ `Unary of 'ud * string * 'ud expr_tree |
                     `Binary of 'ud * string * 'ud expr_tree * 'ud  
expr_tree ]

type 'ud any_tree = [ 'ud type_tree | 'ud expr_tree ]

let ud = function
| `Primitive (ud, _) | `Pointer (ud, _) | `Unary (ud, _, _) | `Binary  
(ud, _, _, _) -> ud

Daniel

[1] http://caml.inria.fr/pub/docs/manual-ocaml/manual006.html#htoc41