The letters s and the dentals when coming in contact with ś and the palatals, are changed to ś and palatals respectively.,
The rule of यथासंख्य does not apply here with regard to first part स्तो श्चुना ॥ The स in contact with श is changed to श but it is also so changed when in contact with letters of च class. Similarly letters of त class coming in contact with श or a letter of च class, are changed to च class. The rule of यथासंख्यं, however, applies to the substitutes, namely स is changed to श, and तु to चु ॥
Ist. स in contact with श : as, वृक्षस् + शेते = वृक्षश्शेते, so also प्लक्षश्शेते ॥
2nd. स in contact with चु :- as, वृक्षस् + चिनोति = वृक्षश्चिनोति, प्लक्षश्चिनोति, वृक्षश्छादयति, प्लक्षश्छादयति ॥
3rd. तु with श :- अग्निचित् + शेते = अग्निचिच्छेते, so सोमसुच्छेते ॥
4th. तु with चु :- अग्निचित् + चिनोति = अग्निचिच् चिनोति, so also सोमसुच्चिनोति, अग्निचिच्छादयति, अग्निचिज्जयति, अग्निचिज् झकारम्, सोमसुच्छादयति, सोमसुज्जयति, सोमसुज्झकारम्, अग्निचिज्जकारः, सोमसुज्जकारः ॥ Similarly मस्ज् gives मज्जति, the स is changed to द् by झलां जश् झशि 8.4.53, and then this द् is changed to a palatal i. e. to ज् here; and द् obtained by जश् rule is not considered asiddha here. See 8.2.3. So also from भ्रस्ज् we have भृज्जति ॥
5th. चकार followed by तकारः as, यज् + न 3.3.90 = यज + ञ् = यज्ञः, याच् + न = याच्ञा ॥ In fact the instrumental case श्चुना shows that the mere contact of स and तु with श and चु is enough to induce the change, whether स्तु is followed by श्चु, or श्चु be followed by स्तु ॥ Other examples of mere contact are :-
5th. (a) स followed by चु is changed into शः as, भ्रस्ज् + ति = भृस्ज् + श + ति 3.1.77 (VI. I. 16) = भृस्ज + ति = भृदज् + ति = 8.4.53 = भृज्जति ॥ Similarly मस्ज forms मज्जति, व्रश्च forms वृश्चति ॥
The aphorism शात् 8.4.44 which prohibits the change of तु into चु when following the letter श, indicates by implication that the rule of mutual correspondence according to the order of enunciation 1.3.10 does not hold good here.
Had the sutra been स्तो श्चोः श्चुः i. e. instead of instrumental, had there been the locative case, then the rule would not have applied to cases covered by the fifth clause.,