that code :

-----------------------------------------------
class ['a] nil = object(self:'a)
    method next = self
    method set_next n = ()
    method apply (f:'a -> unit) (upto:'a) = ()
end;;


class ['a] node = object(self:'a)
    constraint 'a = 'a #nil   (* line 10 *)
    inherit ['a] nil
    val mutable next = new nil
    method set_next n = next <- n
    method apply f upto =
        f self; if self <> upto then self#next#apply f upto
end;;
   
let a = new node;;
-----------------------------------------

causes this exception:
Fatal error: exception Ctype.Unify(_, 0)
(Program not linked with -g, cannot print stack backtrace)


without line 10 it compiles, allthough I get this error-message:
The type of this expression,
< apply : ('a -> unit) -> 'a -> unit; next : 'a;
  set_next : (< apply : ('b -> unit) -> 'b -> unit; next : 'b;
                set_next : '_c -> unit >
              as 'b) ->
             unit >
as 'a, contains type variables that cannot be generalized


is the exception o.k. ? I know now that the contstraint statement is wrong and 
sensless here...

 Michael