oo type question

[Michael Wohlwend <micha-1@fantasymail.de>]

I try to do the following:

I have some objects (all with an id method):
let o1 = object method id = 1  method hi = "hi" end
let o2 = object method id = 2  method ho = "ho" end

and now:

class store = object 
	val mutable  ids = []
	method add o =  ids <- o#id :: ids
end

this doesn't work, since the hidden type variable in the argument to add (I 
think). But why can't the compiler just set o to be of type < obj:int; ..> ?.
So I try:
class ['a] store = object 
	val mutable  ids = []
	method add (o: 'a) =  ids <- o#id :: ids
end;;

...but...

let s = new store ;;
give the type:
val s : < id : '_a; _.. > store = <obj>

so this doesn't work:

s#add o1;
s#add o2

his expression has type < hi : string; id : int > but is here used with type
  < ha : string; id : int >
The second object type has no method ho

If I define a function, say:
let f o = o#id;;
I get :
val f : < id : 'a; .. > -> 'a = <fun>
so this works:
f o1; f o2


why does this not work with methods?

cheers
 Michael