theorem
  Y c= Tarski-Class X & Z c= Tarski-Class X implies [:Y,Z:] c= Tarski-Class X
proof
  assume
A1: Y c= Tarski-Class X & Z c= Tarski-Class X;
  let x,y be object;
  assume [x,y] in [:Y,Z:];
  then x in Y & y in Z by ZFMISC_1:87;
  hence thesis by A1,Th27;
end;
