Praudhamanorama p17

p17

'aśvāḥ': formed from the root aś with the suffix kvan, it has an acute on the initial syllable due to the accent of an n-marker (nit). In continuous text (saṃhitā), however, the sound o is acute by the rule 'A single substitute [for an acute and a non-acute] is acute with an acute' (Pāṇini 8.2.25). 'ye arāḥ': both have the acute on the final syllable by a Phiṭsūtra. But the single substitute is circumflex in the alternative, by the statement: 'Or circumflex when followed by a non-acute at the beginning of a pada' (Pāṇini 8.2.6).
mukhanāsikāvacano'nunāsikaḥ (Pāṇini 1.1.8)