Subtype problem

[Hendrik Tews <tews@tcs.inf.tu-dresden.de>]

Hi,

could somebody tell me, why

-------------------------Version 1---------------------------
class point x = 
  val mutable x = (x : int)
  method x = x
  method move (i : int) =  {< x = x + i >} 
end;;

class colored_point x c =
  val mutable x = (x : int)
  val color = (c : int)
  method x = x
  method move (i : int) = {< x = x + i >} 
  method color = color 
end;;

let p = ((new colored_point 2 1 :> point) : point)
-------------------------Version 1---------------------------

compiles without problem, while

-------------------------Version 2---------------------------
class point x = 
  val mutable x = (x : int)
  method x = x
  method move (i : int) =  Some {< x = x + i >} 
end;;

class colored_point x c =
  val mutable x = (x : int)
  val color = (c : int)
  method x = x
  method move (i : int) = Some {< x = x + i >} 
  method color = color 
end;;

let p = ((new colored_point 2 1 :> point) : point)
-------------------------Version 2---------------------------

produces an error? (The only difference is the move method, which
delivers an option in version 2). The error message is 

-------------------------Error-------------------------------
This expression cannot be coerced to type
  point = < x : int; move : int -> point option >;
it has type
  colored_point =
    < x : int; move : int -> colored_point option; color : int >
but is here used with type
  < x : int; move : int -> point option; color : int >
Type
  colored_point =
    < x : int; move : int -> colored_point option; color : int >
is not compatible with type point = < x : int; move : int -> point option >
-------------------------Error-------------------------------


Bye,

Hendrik