Summary:
- favourite & second-favourite choices: `p.join(q)` or `p.add(q)`.
- most disliked: `p[q]`
`p[q]` -1: looks like indexing or key-lookup, isn't either.
`p + q` +0: potential confusion between path component joining and filename suffix appending.
`p / q` +0: looks funny if either arg is a literal string.
`p.join(q)`: +1: self-explanatory, suggests os.path.join, I am not convinced that Nick's fears about confusing Path.join and str.join will be a problem in practice.
`p.pathjoin(q)` and `p.joinpath(q)` -1: dislike repeating the type name in the method name; also, too easy to forget which one is used.
`p.add(q)` +0.5: nice and short, but not quite self-explanatory.
`p.append(q)` -0: suggests an in-place modification.