reserve x,A,B,X,X9,Y,Y9,Z,V for set;

theorem Th8:
  X c= Z & Y c= Z implies X \/ Y c= Z
proof
  assume
A1: X c= Z & Y c= Z;
  let x be object;
  assume x in X \/ Y;
  then x in X or x in Y by XBOOLE_0:def 3;
  hence thesis by A1;
end;
