text: def repeat_last(initial=None):
      msg, old, new = None, None, initial
      while True:
          msg = yield old
          old, new = new, msg
          print('Old: {} New: {}'.format(new, old))
  
  r = repeat_last('z')
  msgs = [None, 'a', 'b', 'c']
  print('(Priming)')
  for msg in msgs:
      print('Sending: {}'.format(msg))
      res = r.send(msg)
      print('Received: {}'.format(res))
  
  print('Sending: None')
  r.next()
  print('Sending: None')
  r.next()
  