/usr/lib/s9fes/help/subsetp is in scheme9 2010.11.13-2.
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| 1 2 3 4 5 6 7 8 9 10 | 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
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