'Subtract two Durations.
= ERROR CONDITIONS
- If the input rhs is not in the same frame, an error is thrown.
= INPUT VARIABLES
- rhs    The Duration to subtract.
= RETURN VALUE
- Returns the difference of ourselves and the input Duration.'

def __sub__(self, rhs):

    self.checkSameFrame(rhs, u'sub')
    return Duration(self._frame, (self._seconds - rhs._seconds))
