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

def __add__(self, rhs):

    import matplotlib.testing.jpl_units as U
    if isinstance(rhs, U.Epoch):
        return (rhs + self)
    self.checkSameFrame(rhs, u'add')
    return Duration(self._frame, (self._seconds + rhs._seconds))
