theorem
  X (\) (Y (\/) Z) = (X (\) Y) (/\) (X (\) Z)
proof
  thus X (\) (Y (\/) Z) = X (\) Y (\) Z by Th73
    .= (X (\) Y) (/\) X (\) Z by Th56,Th23
    .= (X (\) Y) (/\) (X (\) Z) by Th62;
end;
