theorem Th3:
  Shift(J,card I) c= I ';' J ';' K
proof
  set IJ= I ';' J;
  dom IJ misses dom Shift(K, card IJ) by AFINSQ_1:72;
  then
A1: IJ c= IJ ';' K by FUNCT_4:32;
  Shift(J, card I) c= IJ by FUNCT_4:25;
  hence thesis by A1,XBOOLE_1:1;
end;
