[wild_preference]

Then you end up with a system where someone can arrive at the right answer via AST that you didn't expect, which was a frustration when I helped students with a service like (IIRC) CodeCademy.

[cellularmitosis]

Parsing the AST is something I would love to apply to a site like 4clojure.com, to get a histogram of the "shape" of all of the submitted solutions.