Simplify abstraction, now that it only deals with lists.
Simplify abstraction, now that it only deals with lists.

Also convert listof/non-empty-listof to create chaperone contracts
when appropriate.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket