Various repairs --- especially to avoid duplicated prose.
Instead of
@(define explain @t{Long explanation about X...})
@defproc[(a ....) ....]{ .... @|explain| }
@defproc[(b ....) ....]{ .... @|explain| }
@defproc[(c ....) ....]{ .... @|explain| }
write
@defproc[(a ....) ....]{ .... Long explanation about X ... }
@defproc[(b ....) ....]{ .... X is like @racket[a]. }
@defproc[(c ....) ....]{ .... X is like @racket[a]. }
Otherwise, a reader is forced to reverse-engineer the
abstraction underlying `a', `b', and `c'.
