<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def focus_first_child(self):
 """move focus to first child of currently focussed one"""

        w, focuspos = self.get_focus()
        child = self._tree.first_child_position(focuspos)
        if child is not None:
            self.set_focus(child)