let X, Y, Z be set ; :: thesis: ( X c= Z & Y c= Z implies X \/ Y c= Z )
assume A1: ( X c= Z & Y c= Z ) ; :: thesis: X \/ Y c= Z
let x be object ; :: according to TARSKI:def 3 :: thesis: ( not x in X \/ Y or x in Z )
assume x in X \/ Y ; :: thesis: x in Z
then ( x in X or x in Y ) by XBOOLE_0:def 3;
hence x in Z by A1; :: thesis: verum