plt/racket — pull request #854 by gus-massa (2014-12-29 12:06:28+00:00)
Title: Fix is_arity_list

The first commit fixes `is_arity_list` and a small error in its documentation. I was not sure where I should put the test cases, I thought that `basic.rktl` was a good place.

The seccond (unrelated) commit adds more test cases of disjoint types, and it tests automatically that each of the examples belongs only to one disjoint type.