After a word ending in n , dhuṭ is optionally the augment to a word beginning with s,
Thus भवान् त्साये, महान् त्साये or भवान् साये, महान् साये ॥ The ध् of the augment becomes त् by 8.4.55. This त् is asiddha 8.2.1, and therefore त् is not changed to रु by 8.3.7. This is the reason why the augment is exhibited as धुट् and not तुट् ॥ In sutra 8.3.29 तुट् would have done as well but not so here. This view of the Kasika, however, is not approved by Padamanjari. The न् can never be changed to रु here, because त् is followed by स् which is not an अम् letter. The word अम्परे is understood in 8.3.7. In fact, तुट् would have been a better augment.,