Allows a subelement of a complex element to notify its root element that it has been changed. 

Here's how it's supposed to work: 

1) The subelement class's close() calls its triggers a "modified" notification which calls its xmitPropagateModify(). 2) The subelement class calls the its parent's recvPropagateModify() its element's pointer. 3) The owner's class's recvPropagateModify() sends a "modified" notification to the top of its class. 

The default implementation of this function does nothing. This function can be overridden in custom classes.