<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _box_click(self, event):
 """Check or uncheck box when clicked."""

        x, y, widget = event.x, event.y, event.widget
        elem = widget.identify("element", x, y)
        if "image" in elem:
            # a box was clicked
            item = self.identify_row(y)
            if self.tag_has("unchecked", item) or self.tag_has("tristate", item):
                self._check_ancestor(item)
                self._check_descendant(item)
            else:
                self._uncheck_descendant(item)
                self._uncheck_ancestor(item)