I see your point, but this isn't the case.

At this point we know that the ```CubeSignature```s are compatible. Therefore, this means that the cubes have an **equal** number of ```DimCoords``` with matching meta-data **and** the same number of dimensions. So it's safe to ```enumerate``` here. An ```IndexError``` will never be raised.

I've subsequently made this constraint explicit within the ```CubeSIgnature```.