code: public void retireWindow(W window) {
  		W removed = this.mapping.remove(window);
  		if (removed == null) {
  			throw new IllegalStateException("Window " + window + " is not in in-flight window set.");
  		}
  	}
docs: Removes the given window from the set of in-flight windows.
  
  @param window The {@code Window} to remove.