reserve i,j,e,u for object;
reserve I for set; 
reserve x,X,Y,Z,V for ManySortedSet of I;

theorem
  (X (\+\) Y) (\) Z = (X (\) (Y (\/) Z)) (\/) (Y (\) (X (\/) Z))
proof
  thus (X (\+\) Y) (\) Z = (X (\) Y (\) Z) (\/) (Y (\) X (\) Z) by Th72
    .= (X (\) (Y (\/) Z)) (\/) (Y (\) X (\) Z) by Th73
    .= (X (\) (Y (\/) Z)) (\/) (Y (\) (X (\/) Z)) by Th73;
end;
