Definition: invalid; of suspended validity for the time being: not functioning for the time being. The term is frequently used in Pāṇini's system of grammar in connection with rules or operations which are prevented, or held in suspense, in connection with their application in the process of the formation of a word. The term (असिद्ध) is also used in connection with rules that have applied or operations that have taken place, which are, in certain cases, made invalid or invisible as far as their effect is concerned and other rules are applied or other operations are allowed to take place, which ordinarily have been prevented by those rules which are made invalid had they not been invalidated. Pāṇini has laid down this invalidity on three different occasions (1) invalidity by the rule पूर्वत्रासिद्धम् VIII.2.1. which makes a rule or operation in the second, third and fourth quarters of the eighth chapter of the Aṣṭādhyāyī invalid when any preceding rule is to be applied, (2) invalidity by the rule असिद्धवदत्राभात् which enjoins mutual invalidity in the case of operations prescribed in the Ābhīya section beginning with the rule असिद्धवत्राभात् (VI. 4.22.) and going on upto the end of the Pāda (VI.4.175), (3) invalidity of the single substitute for two letters, that has already taken place, when ष् is to be substituted for स्, or the letter त् is to be prefixed, cf. षत्वतुकोरसिद्धः (VI. 1.86). Although Pāṇini laid down the general rule that a subsequent rule or operation, in case of conflict, supersedes the preceding rule, in many cases it became necessary for him to set, that rule aside, which he did by means of the stratagem of invalidity given above. Subsequent grammarians found out a number of additional cases where it became necessary to supersede the subseguent rule which they did by laying down a dictum of invalidity similar to that of Pāṇini. The author of the Vārttikas, hence, laid down the doctrine that rules which are nitya or antaraṅga or apavāda, are stronger than, and hence supersede, the anitya, bahiraṅga and utsarga rules respectively. Later gram marians have laid down in general, the invalidity of the bahiraṅga rule when the antaraṅga rule occurs along with it or subsequent to it. For details see Vol. 7 of Vvyākaraṇa Mahābhāṣya(D. E. Society's edition) pages 217-220. See also Pari. Śek. Pari. 50.