We now use induction on p. Suppose Y = [Xo, X1}U. . . Us YP-1, XP). From the above,. [Xo,. Xe]. C. N. (IXo,. X-1]U. [X,. I,. X,j. C NE. (NG-1)*([Xo,. X1] U - - - U .