Add iseqss to iset.mm
Add iseqss to iset.mm

It is quite possible there is more we can do about reducing the
number of hypotheses to this theorem, but this is at least a
start which should take care of some of the obvious cases (such
as S = { 0 } and T = CC ).

metamath/set.mm,metamath/set.mm,metamath/set.mm