  We study the behaviour of iterations of the difference operator delta on
streams over {0,1}. In particular, we show that a stream sigma is eventually
periodic if and only if the sequence of differences sigma, delta(sigma),
delta(delta(sigma)), ..., the `delta-orbit' of sigma as we call it, is
eventually periodic. Moreover, we generalise this result to operations delta_d
that sum modulo 2 the elements of each consecutive block of length d+1 in a
given 01-stream. Some experimentation with delta-orbits of well-known streams
reveals a surprising connexion between the Sierpinski stream and the Mephisto
Waltz.
