Delegator ::= Object clone tap {
  self delegate := Nil.
  self clone := {
    Parents above (parent self, 'clone) call tap {
      self delegate := #'(self delegate).
    }.
  }.
  self operation := {
    localize.
    if { this delegate slot? 'thing. } then {
      this delegate thing.
    } else {
      "default implementation".
    }.
  }.
}.

Delegate ::= Object clone tap {
  self thing := "delegate implementation".
}.

;; No delegate
foo := Delegator clone.
println: foo operation. ;; "default implementation"

;; Delegate which lacks `thing`
foo delegate := Object.
println: foo operation. ;; "default implementation"

;; Delegate which implements `thing`
foo delegate := Delegate.
println: foo operation. ;; "delegate implementation"