1st Rule
The final letters "s" and "r" may be changed to Visarga (ḥ). There are words "originally" ending in "s" and words "originally" ending in "r", but Visarga may be substituted for the final "s" and "r" according to the following sub-rules: a) Final "s" of a word followed by any letter or by nothing. b) Final "r" of a word followed by a hard consonant or by nothing. Of course, once "ḥ" is substituted for "s" or "r", you might have to apply other rules to the word ending now in Visarga.