S9 LIB  (subset? list1 list2 ...) ==>  list

        (load-from-library "subsetp.scm")

Check whether each of a sequence of sets is a subset of the subsequent
set in the list. The test succeeds even if a set is an improper subset
of (i.e. the same sets) the subsequent one.

(subset? '(a) '(a b) '(a b) '(a b c d))  ==>  #t
(subset? '(a b c))                       ==>  #t
