The third fused negative, and the rule it quietly breaks
Two Serbian verbs fuse не into a single word instead of writing it separately - everyone who has met нисам and немам knows this much. A third joins them once Futur I enters the picture, and all three share something more useful than a spelling rule: fusing the negation strips the enclitic status right off the word, which is exactly why a sentence is allowed to start with one of them.