The guṇa is the single substitute of the final a or ā of a preceding word and the simple vowel of the succeeding (a or ā + a vowel = guṇa),
The word अचि is understood here. For the vowel which follows an अ or आ, and for the अ or आ which precedes a vowel, in the room of both these preceding and succeeding vowels, there is the single substitute guna. Thus तव + इदम् = तवेदम्, खट्वा + इन्द्रः = खट्वेन्द्रः, माला + इन्द्रः = मालेन्द्रः, तव + ईहते = तवेहते, खट्वा + ईहते = खट्वेहते, तव + उदकम् = तवोदकम्, खट्वा + उदकम् = खट्वोदकम्, तव + ऋश्यः = तवर्श्यः, खट्वा + ऋश्यः = खट्वर्श्यः, तव + लृकारः = तवल्कारः, खट्वा + लृकारः = खट्वल्कारः ॥ By analogy of 1.1.51, the guna substitute of लृ being अ is always followed by ल्, as that of ॠ is followed by र् ॥ This universal rule is limited by the following aphorism.,