For mathematical fiction, I've found nothing beats macroeconomics textbooks.

The system we used here was actually smart enough to not flag common idioms like this, and also still flag cases where only the variable names and formatting had been changed. Also, our assignments were usually complex enough that there would be enough variation, anyways. Most things about our automated hand-in system sucked, but the plagiarism detector wasn't one of them. If you're curious: https://www.ipd.uni-karlsruhe.de/jplag/

Eh, that makes sense for an online poll with write-in. It discourages 4chan et all coming along and flooding the poll suggesting the node should be named something along the lines of "NIGGER COCKS OLOLOL".