theorem
  X (\) Y c= Z implies X c= Y (\/) Z
proof
  assume
A1: X (\) Y c= Z;
  X (/\) Y c= Y by Th15;
  then X (/\) Y (\/) (X (\) Y) c= Y (\/) Z by A1,Th20;
  hence thesis by Th65;
end;
