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

theorem Th19:
  Z c= X & Z c= Y implies Z c= X /\ Y
proof
  assume
A1: Z c= X & Z c= Y;
  let x be object;
  assume x in Z;
  then x in X & x in Y by A1;
  hence thesis by XBOOLE_0:def 4;
end;
