ur is substituted for the long ṝ of a root when it is preceded by a labial consonant belonging to the root.,
As पूर्त and पुपूर्षति from पॄ, so also मुमूर्षति ॥ The lengthening is by 8.2.77. The rule applies when the dento-labial व precedes: as वुवूर्षति ऋत्विजम् from वॄ; so also प्रावुवूर्षति कम्बलम् ॥ The labial consonant must be the consonant of the root. Therefore when ॠ 'to go' is preceded by सम्, the rule will not apply, for म् is not part of the root: as समीर्णम् by 7.1.100.
Vart:- The Guna and Vriddhi do take place in supersession of इर् and उर् substitution. Thus आस्तरणम् and आस्तारकः (from स्तॄ with ल्युट् and ण्वुल्), निपरणम्, निपारकः from पॄ, निगरणम्, निगारकः from गॄ ॥,