What should it do when it gets a relative pathname? E.g.: #p"foo/bar/".
I think #p"/" should not be treated fundamentally different to #p"foo/".
E.g.: (unmerge-pathnames #p"foo/bar/baz" #p"foo/bar/") => #p"baz" (unmerge-pathnames #p"/foo/bar/baz" #p"foo/bar/") => #p"/foo/bar/baz"