The seṭ niṣṭhā affix is optionally not kit after the verbs with a penultimate u if used impersonally or denoting the beginning of action.,
According to some the set Nishtha suffixes are not kit after the verbs having a penultimate उ if those verbs are used impersonally (bhava) or denote the beginning of action (adi-karman). Thus द्युत् 'to shine', द्युतितमनेन or द्योतितमनेन 'it has been lighted.' Similarly प्रद्योतितः or प्रद्युतितः 'he first illumined.' The word penultimate or upadha has been defined in sutra 1.2.65 ante.
If the verb has any other vowel than उ as penultimate, then the rule does not apply. As लिखितमनेन 'written by him.' Here इ is penultimate; and therefore no option is allowed.
This rule has no application where the Nishtha does not express either भाव 'condition' or 'Impersonal action'; or आदिकर्म 'if beginning of action', but is used as a verbal adjective. As रुचितं कार्षापाणं ददाति 'he gives a shining coin (called karshapana).' Here the word रुचित 'shining' is used as adjective.
The rule applies to the Nishtha which takes the intermediate इ and not to others as प्रभुक्त ओदनः 'the rice that has been eaten.' Here no option is allowed.
This optional form of past participle is restricted in its scope. Those verbs which take the vikarana शप् i.e., the verbs of the Bhu class &c., admit of this option. Thus गुध् 'to wrap up' belongs to Divadi class, and therefore, its participle has one form only as गुधितं.,