bookvol10.2 bug 7247: subset? uses wrong length test, should use <=

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bug 7247: subset? uses wrong length test, should use <=

SI ==> Set Integer
SSI ==> Set SI
si:SI := set [2]      -- {2}
subset?(si,si)        -- true
ssi:SSI := set [si]   -- {{2}}
subset?(ssi,ssi)      -- false (a bug)

is fixed by changing
   subset?(s, t)   == #s < #t and _and/[member?(x, t) for x in parts s]
to
   subset?(s, t)   == #s <= #t and _and/[member?(x, t) for x in parts s]
in category FSAGG FiniteSetAggregate

Bug report by Ralf Hemmecke, fix by Waldek Hebisch