What if you're dropping an axis that is not separable from another?  You're bound to end up with something that doesn't make sense.

For example, RA usually depends on both x and y.  If you drop x, how can this give you the correct result.

wcslib already has a function to do this (`wcssub`, wrapped as `Wcsprm.sub`) that will raise an exception if trying to remove a non-separable axis.