greghendershott/rackjure — pull request #24 by greghendershott (2014-01-28 20:35:30+00:00)
Title: `box-swap!` should error in Racket < 5.92. Closes #6.

On Racket < 5.92, it can't work properly -- there's an intermittent bug -- so better to have it be a function that explicitly errors when called.