-- >>> let (Right r) = parse subdomain "" "www.example.com"
-- >>> pretty_print $ subdomain_label_neighbors r
-- ["(\"www\",\"example\")","(\"example\",\"com\")"]
-- >>> let (Right r) = parse subdomain "" "www.example.com"
-- >>> pretty_print $ subdomain_label_neighbors r
-- ["(\"www\",\"example\")","(\"example\",\"com\")"]