theorem Th19:
  (R |_2 X) |_2 Y = R |_2 (X /\ Y)
proof
  thus (R |_2 X) |_2 Y = R /\ ([:X,X:] /\ [:Y,Y:]) by XBOOLE_1:16
    .= R |_2 (X /\ Y) by ZFMISC_1:100;
end;
