/**
  \example forward.cpp 

  The sketch in \ref pingpong.cpp responded the same way to 'ping'
  packets arriving on any face.  This sketch shows a way of responding
  more narrowly, only when certain types of packets occur on certain
  faces.

  In this sketch, any type 'd' packet arriving on the east face is
  forwarded out the west face, and vice-versa.  But 'd's arriving from
  the north or south are simply ignored.  

 */
