It was surprising to me that arithmetic operations with `Longitude` all downgrade to `Angle` in exactly the way I hoped.  I initially worried I would have to implement type rules in `__add__` and so forth.  But from that basis I'm not surprised that inplace operations work:
```
In [2]: lon = Longitude ('725d')

In [3]: lon
Out[3]: <Longitude 5d00m00.00000s>

In [4]: lon += Longitude('725d')

In [5]: lon  # Now an unbounded / unwrapped Angle
Out[5]: <Angle 10d00m00.00000s>

In [6]: Longitude('725d') + Longitude ('359d')
Out[6]: <Angle 369d00m00.00000s>
```