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

theorem Th28:
  X c= Y implies X /\ Y = X
proof
  assume
A1: X c= Y;
  thus X /\ Y c= X by Th17;
  let x be object;
  assume
A2: x in X;
  then x in Y by A1;
  hence thesis by A2,XBOOLE_0:def 4;
end;
