1) If we don't add "str", we get nothing (no replacement at all) when the recursion level limit is excedeed, and perhaps some of the replacements found is good. Useful just for some extreme cases.
2) The limit (4, 5, 6...) is "subjective" (or language-dependent). If the replacement keys in "language.info" are frequent in words, then you will need more levels. I was using s->ss in Catalan, but I have removed it because it sometimes created too much possible replacements.