deriv SD cv (898) ashtadhyayi.com hei.de L 898 ETT STT a 7.2.9 ALPH OLDHOMEPAGE NEWHOMEPAGE

ti;tu;tra;ta;tha;si;su;sara;ka;seSu ca

तिॱतुॱत्रॱतॱथॱसिॱसुॱसरॱकॱसेषु च ONPANINI 72009

The /kRt affixes ktic क्तिच् tun तुन् /STran tan तन् kthan क्थन् ksi क्सि ksu क्सु saran सरन् kan कन् sa स don't get /iT.

As pat पत् is a /seT root, we have pat पत् + /tRc → patitR- पतितृॱ, but with STran ष्ट्रन् affix we get...

pat पत् + STran ष्ट्रन् → pattra- पत्त्रॱ

...and not patitra-.

Also, lU लू is a /seT root (lavitum लवितुम्), but with tan तन् it makes lota- लोतॱ, not lavita-.

This rule has some exceptions —

lU लू + /tran → !**lavitra- लवित्रॱ n "reaping tool", "sickle", "scythe"

, see candravasu for more. When a rule does not work 100% of the time, like this one, grammarians either make a /vArttika, or make lists of exceptions, or just say to the hell with it and declare it to be an /anitya ("inconstant", "unreliable") rule. That'd be a rule that mostly works and seems to be compulsory, but might as well not be there or be optional.