(char-set-contains? (char-set-union
                        (char-set #\& #\;)
                        (char-set-delete
                         (char-set-complement (string->char-set
   "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"))
                         #\space)) #\?)

=>

#f

but:

(char-set-contains? (char-set-union
                        (char-set-delete
                         (char-set-complement (string->char-set
   "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"))
                         #\space)) #\?)

=>

#t

(member #\? (char-set->list (char-set-union
                        (char-set #\& #\;)
                        (char-set-delete
                         (char-set-complement (string->char-set
   "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"))
                         #\space))))

=>

#f


But:

(member #\? (char-set->list (char-set-union
                        (char-set #\&)
                        (char-set-delete
                         (char-set-complement (string->char-set
   "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"))
                         #\space))))

takes forever