karl> (def gizmo (atom[])) 
#'karl/gizmo
karl> (add-watch gizmo :dog 
        (fn [key gizmo old new]
          (println key old new)))
#<Atom@5f5a940a: []>
karl> (swap! gizmo conj true)
:dog [] [true]
[true]
karl> (if (seq? (seq @gizmo))
        (swap! gizmo pop))
:dog [true] []
[]
karl> (if (seq? (seq @gizmo))
        (swap! gizmo pop))
nil