Once again, why not just type:

```python
>>> v = 4*R1.x + 5*R1.z
>>> v.express(R0)
```

Why does this constitute a method on ReferenceFrame. If we think about the vector as a thing, we can say "Express this vector in another reference frame". But it thinking of a reference frame as a thing, it doesn't make sense to say "Express this reference frame in another reference frame" or "Express this reference frame with this vector". So why should the reference frame have a method called express?

Seems like this could be a standalone function but doesn't make much sense to me why it is a method of a reference frame.

Or maybe the method name could be better, like "R0.express_vector_in(vector)".