def on_mouse_enter(self, x, y):
<EOL>children_under_mouse = self.__find_children_under_mouse(x, y)<EOL>if self.propagate_mouse_events:<EOL><INDENT>for child in children_under_mouse.entered:<EOL><INDENT>child.dispatch_event('<STR_LIT>', x, y)<EOL><DEDENT><DEDENT>self.__update_rollover_state('<STR_LIT>', x, y)<EOL>
React to the mouse crossing into this widget.

The ``on_mouse_enter`` event is propagated to any children under the 
mouse and an ``on_rollover`` event is triggered to indicate that the 
widget's rollover state is now "over".