Yeah, the language should be stricter for clarity. You're right in that a candidate axis is returned iff a *single* candidate axis is found, otherwise ```None```.

This is exactly the behaviour that we want. There should be only ever one unambiguous dimension over which to concatenate. In fact, I believe it's the case that there can only ever be two states here: 
 * no candidate dimension
 * one candidate dimension
I can't bend my mind to create a scenario where two cubes can validly concatenate over more than one dimension ... can you?