ऋतश्च संयोगादेर्गुणः

A root ending in short ऋ and preceded by a conjunct consonant gets गुण in the लिट् (Perfect Tense).
As सस्वरतुः, सस्वरुः from स्वृ, दध्वरतुः, दध्वरुः from ध्वृ, सस्मरतुः, सस्मरुः from स्मृ ॥ Why do we say ending in ऋ? Observe चिक्षियतुः, चिर्क्षियुः ॥ Why do we say beginning with a conjunct consonant? Observe चक्रतुः, चक्रुः ॥ This sutra ordains Guna where there was prohibition by 1.2.5 read with 1.1.5. But this does not debar the Vriddhi caused by णल् 7.1.115. In fact, that prior rule supersedes this posterior rule, as सस्वार, दध्वार, सस्मार ॥ The word लिटि is to be supplied in the sutra, the rule does not apply to Nishtha &c., as स्मृतः, स्मृतवान् ॥

Vart:- For the sake of कृञ्, the guna should be stated even where the double-consonant is in the penultimate position and not in the beginning. As संचस्करतुः, संचस्करुः ॥ See 6.1.135. On the maxim पूर्वं धातुः साधनेन युज्यते पश्चादुपसर्गेण 'a root is first developed fully and then the preposition is added to it'; we first develop कृ in the Perfect by reduplication, which gives us चकृ + अतुस्, then we add the preposition as संचकृ + अतुः, then we add सुट् though the reduplicate intervenes, by 6.1.136 and 6.1.137, as संचस्कृ + अतुः, now the root assumes a form in which the penultimate begins with a double consonant, and applying the vartika we make guna, and get संचस्करतुः ॥

It is by this consideration that in संस्कृषीष्ट, उपस्कृषीष्ट, the सुट् augment being Bahiranga and consequently considered as non-existent (asiddha), there
is no इट् augment added by 7.2.43.