reactfx/src/main/java/org/reactfx/ProxyObservable.java
@Override
    public final Subscription observe(O observer) {
        P adapted = adaptObserver(observer);
        underlying.addObserver(adapted);
        return () -> underlying.removeObserver(adapted);
    }
@Override
    public final Subscription observe(O observer) {
        P adapted = adaptObserver(observer);
        underlying.addObserver(adapted);
        return () -> underlying.removeObserver(adapted);
    }
Overridden to avoid second transformation on removeObserver