let X be set ; :: thesis: for Y being integer-membered set st X c= Y holds
X is integer-membered
let Y be integer-membered set ; :: thesis: ( X c= Y implies X is integer-membered )
assume A1: X c= Y ; :: thesis: X is integer-membered
let x be set ; :: according to MEMBERED:def 5 :: thesis: ( x in X implies x is integer )
thus ( x in X implies x is integer ) by A1; :: thesis: verum