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 (\/) Z) (\/) (Y (\/) Z)
proof
  thus X (\/) Y (\/) Z = X (\/) (Y (\/) (Z (\/) Z)) by Th28
    .= X (\/) (Z (\/) Y (\/) Z) by Th28
    .= (X (\/) Z) (\/) (Y (\/) Z) by Th28;
end;
