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

theorem Th78:
  (X (\) Y) (\) Y = X (\) Y
proof
  thus (X (\) Y) (\) Y = X (\) (Y (\/) Y) by Th73
    .= X (\) Y;
end;
