def METHOD_NAME(self, index):
    """
    Descend into call tree for this node.
    """
    expanding_item = self._model.itemFromIndex(index)
    if not expanding_item.populated:
        dbg = self.instance.debugger_mgr.debugger
        if dbg.am_none:
            return
        called = dbg.get_called_functions(expanding_item.event)
        for func_or_addr, event in called:
            expanding_item.appendRow(CallTreeItem(func_or_addr, event))
        expanding_item.expandable = len(called) > 0
        expanding_item.populated = True