Māṇḍūkī Śikṣā 58

58

svaritātparāṇī yāni syuranudāttānyudāttavat
sarvāṇi pracayaṃ yānti hyupodāttaṃ na vidyate

Those grave accents which follow a circumflex accent become like the acute accent; / all pass into the monotone accent, and an sub-acute accent is not found.