विप्रतिषेधे परं कार्यम्

When rules of equal force prohibit each other then the last in the order herein given is to take effect.
The word विप्रतिषेधे means 'opposition of rules of equal force.' When two topics having different objects in view find scope of action simultaneously in one particular case, that opposition of equal forces is called vipratishedha. A general rule (utsarga) and its exception (upavada), or an invariable (nitya) and an optional (anitya) rule, or an antaranga and a bahiranga rule, are not rules of equal force. There the stronger prevails against the weaker. As an example of rules of equal force, see (VII.3.102.) and 7.3.103. The first rule declares, 'when a case-affix beginning with a letter of yan pratyahara follows, the long vowel is substituted for the final of an inflective base ending in a short अ.' As Vriksha + bhyam = Vrikshabhyam. The next rule declares :- When a plural case-affix beginning with a letter or jhal pratyahara follows, ए is the substitute for the final short अ of an inflective base.' As Vriksha + su = Vriksheshu. But when the plural case-affix bhyah follows, what rule are we to apply? For the letter bha belongs both to the pratyaharas yan and jhal. Are we to lengthen the short अ, or substitute ए? The present sutra gives the reply, ए is to be substituted because 7.3.103 ordaining ए follows next to 7.3.102. Thus Vriksha + bhyah = Vrikshebyah.