let X be set ; :: thesis: ( X is integer-membered implies X c= INT )
assume A1: X is integer-membered ; :: thesis: X c= INT
let x be object ; :: according to TARSKI:def 3 :: thesis: ( not x in X or x in INT )
assume x in X ; :: thesis: x in INT
then x is integer by A1;
hence x in INT ; :: thesis: verum