You discovered a backwards incompatible change to Redex. Changing M and N to A and B is a good fix. (The errata really should talk about this.)
Here are two related threads: https://groups.google.com/d/msg/racket-users/be54SG881GU/bndA4eiGAQAJ https://groups.google.com/d/msg/racket-users/NMuBaVlcDAU/v0WyrxZbDQAJ -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
