The affix ani comes after a verb when the particle nañ is in composition with it and the sense implied is that of 'cursing'.,
The anuvritti of the word 'optionally' does not extend to this aphorism. The word आक्रोश means to curse. This supersedes the affixes क्तिन् &c. Thus अकरणिस्ते वृषल भूयात् 'may failure be to thee, O sinner'! तस्याकरणिरेवास्तु 'may he be disappointed'.
Why do we say 'when meaning to curse'? Observe अकृतिस्तस्य कटस्य 'the non-making of his mat'.
Why do we say 'when नञ् is in composition'? Observe सृतिस्ते वृषल भूयात् 'may death be to thee, O! sinner'!,