Plagiarism Detection for Lisp

Computers made it very easy to copy someone else's work.  This makes grading a difficult task, as the teacher that wants to prevent plagiarism needs to compare each student's assignment against every other student's assignment, a quadratic process that is impractical when the number of assignments gets large. Students know this and some take advantage of it. To be able to detect plagiarism among students' programming assignment we created a software tool that checks all assignments against each other, searching for copied fragments.  Unlike many other tools, this search is not based on textual comparisons or hashing functions but, instead, on collecting pieces of evidence for and against a plagiarism verdict.  This collection is determined by specialized procedures, invoked in a data-driven fashion, that incorporate expert knowledge regarding what is plagiarism and what is not plagiarism. The tool has been successfully used since 1995 in the evaluation of assignments programmed in Lisp dialects, particularly, Common Lisp, Scheme, Racket, and AutoLisp, and its mere existence became a deterrent for plagiarism.