How to destroy particels that are off screen

The title says it. I saw I can set the lifetime but how can I destroy particles that are offscreen?  

box2d has  

```
bool ofxBox2dBaseShape::shouldRemoveOffScreen(ofPtr<ofxBox2dBaseShape> shape) {
    return !ofRectangle(0, 0, ofGetWidth(), ofGetHeight()).inside(shape.get()->getPosition());
}
```

How can I add this to liquidFun?
