Added singleton_iff, Sigma_empty1, Sigma_empty2, Collect_simps
and UnInt_simps to ZF_ss. Added consI1 to ZF_typechecks.
Arith = FOL +
classes arith < term
consts "0" :: "'a::arith" ("0")
"1" :: "'a::arith" ("1")
"+" :: "['a::arith,'a] => 'a" (infixl 60)
end