uses objectclass;
define :class Delegator;
    slot delegate = false;
enddefine;

define :class Delegate;
enddefine;

define :method thing(x : Delegate);
   'delegate implementation'
enddefine;

define :method operation(x : Delegator);
if delegate(x) and fail_safe(delegate(x), thing) then
   ;;; Return value is on the stack
else
   'default implementation'
endif;
enddefine;

;;; Default, without a delegate
lvars a = newDelegator();
operation(a) =>

;;; a delegating to itself (works because Delegator does not
;;; implement thing)
a -> delegate(a);
operation(a) =>

;;; delegating to a freshly created Delegate
newDelegate() -> delegate(a);
operation(a) =>