o coverage:
  o relative($tobase);
  o parts/slice
  o dir->map(..., $n/[$m,$n])
  o File::Fu->program_name
  o File::Fu->program_dir
? directory mod within filename
  - wav/thing/foo.wav => flac/thing/foo.flac
    $file->dirname->map(sub {s/wav/flac/}, 0);
    $file->dirname->map([wav => 'flac'], 0);
    $file->dirname->map([qw(wav flac)], 0);
    ($file &= λ{s/wav/flac/})->dirname &= λ{s/wav/flac/};
    $file->map(λ{s/\.wav$/flac/})->dirname->map(λ{s/wav/flac/}, 0);
x $dir + 'foo/bar' problem
o freedesktop
  o home
  o mimeinfo
  o basedir
o resolve() symlink chase-down
o URI support?
  - is that just a subclass with a volume-like thing?