And the substitute which is indicatory ṅ (even though it consists of more than one letter) takes the place of the final letter only of the original expression.,
This sutra is an exception by anticipation to sutra 1.1.55. By that all substitutes, consisting of more than one letter, replace the whole word.
Thus in forming the dvandva compound of मातृ and पितृ there is a rule 6.3.25 by which it is declared:- 'In the place of words ending in ॠ there is the substitution of आनङ् in forming dvandvas.' Now the substitute आनङ् is a ङित् substitute, and therefore it applies to the last letter of the word and not to the whole word, vis, the ॠ of मातृ is only changed into आ and not the whole word; and we have the compound मातापितरौ, so also होतापोतारौ.
There is an exception, however, in the case of the substitute तातङ् 7.1.35 which, though it has an indicatory ङ्, yet replaces the whole expressions तु and हि and not only their finals. As जीवतात् 'may he live, or may you live'; instead of जीवतु 'may he live,' or जीव 'live thou.',