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

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