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

theorem
  X (\) Y = Y (\) X implies X = Y
proof
  assume X (\) Y = Y (\) X;
  hence X = Y (\) X (\/) Y (/\) X by Th65
    .= Y by Th65;
end;
