एकः पूर्वपरयोः

From here up to 6.1.111 inclusive is always to be supplied the phrase 'for the preceding and the following is substituted'.
This is an adhikara sutra. In every sutra upto 6.1.112 (excluding the last), whatever we shall teach, there in the room of the two, namely, the preceding and the succeeding, it should be understood, that the substitution is one. These form the well known rules of ekadesa, one letter or form replacing two consecutive letters &c. Thus 6.1.87 teaches 'There is guna substitution, when अ or आ is followed by a vowel'. We must supply into that sutra the phrase एकः पूर्वपरयोः i. e. one guna is the substitute for the final अ or आ and the initial vowel. Thus खट्वा + इन्द्रः = खट्वेन्द्रः ॥ Here ए is the single substitute of the both preceding Ietter आ and the succeeding letter इ ॥ The words पूर्व पर show that the substitute operates simultaneously on both. Otherwise the substitute would have come in the place of one only or of each one separately. Thus in आद् गुणः (अचि) = "after अ or आ, there is guna, in a vowel". Here आत् is in this ablative, and by 1.1.67 the guna operation would have taken place on the letter following it: so also अचि is in the Locative and by 1.1.66, the guna operation would have taken place on the preceding; so it is not clear of what letter there should be guna substitution, of the preceding or the succeeding. But the present rule shows it must operate on both simultaneously. The word एक 'single', shows that a separate or a different substitute does not operate: i. e. the substitute must be one, there should not be two separate substitutes one for each sthani, such as we find in the sutra 8.2.42 (रदाभ्यां निष्ठातो नः, पूर्वस्य च दः "न is the substitute of the त of nishtha, after र् and द, and of the preceding द"). Here न् is taught as substitute both of त् and द्, and as एक is not used in the sutra, we get two न, as भिद् + त = भिन् + न = भिन्नः ॥ But this is not the case here. According to Mahabhashya the word एक is redundant.