The acute accent is optionally on the first syllable when a Personal-ending, being sārvadhātuka tense affix beginning with a vowel, provided that the vowel is not the augment iṭ , follows after svap etc., or after hiṃs,
The phrase लसार्वधातुके in the locative case is understood here. Thus स्व꣡पन्ति or स्वप꣡न्ति, श्व꣡सन्ति or श्वस꣡न्ति, हिं꣡सन्ति or हिंस꣡न्ति ॥ The accent on the middle falls by the accent of the affix 3.1.3. Why do we say 'before an affix beginning with a vowel'? Observe स्वप्या꣡त्, हिंस्या꣡त् ॥ Why do we say 'not taking the augment इट्?' Observe स्वपितः꣡ and श्वसितः꣡ ॥ This rule applies to those vowel-beginning affixes which are ङित्; it does not apply to स्व꣡पानि, हिन꣡सानि ॥,