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

theorem Th77:
  (X (\/) Y) (\) (X (/\) Y) = (X (\) Y) (\/) (Y (\) X)
proof
  thus (X (\/) Y) (\) (X (/\) Y) = X (\) (X (/\) Y) (\/) (Y (\) (X (/\) Y))
          by Th72
    .= X (\) Y (\/) (Y (\) (X (/\) Y)) by Th70
    .= (X (\) Y) (\/) (Y (\) X) by Th70;
end;
