Sentences and Sentence Fragments

Typical English Sentence's Syntax: Noun Phrase + Verb Phrase

"I shouted 'Snake!'"

Acceptable English Fragment in Dialogue:  Noun Phrase [+Verb Phrase Understood]

"Snake!" [run like hell!]

Unacceptable English Fragment in Academic Prose Context:

"James Joyce's Ulysses formed many twentieth-century writers' stylistic tastes, for better or worse.  Snake."

"Zookeepers often lose animals if they leave cage doors open.  Snake."