code: def msg2agent(msg, processor=None, legacy=False, **config):
      
  
      if processor.agent is not NotImplemented:
          return processor.agent(msg, **config)
      else:
          usernames = processor.usernames(msg, **config)
          # usernames is a set(), which doesn't support indexing.
          if usernames:
              return usernames.pop()
  
      # default to None if we can't find anything
      return None
docs: Return the single username who is the "agent" for an event.
  
      An "agent" is the one responsible for the event taking place, for example,
      if one person gives karma to another, then both usernames are returned by
      msg2usernames, but only the one who gave the karma is returned by
      msg2agent.
  
      If the processor registered to handle the message does not provide an
      agent method, then the *first* user returned by msg2usernames is returned
      (whether that is correct or not).  Here we assume that if a processor
      implements `agent`, then it knows what it is doing and we should trust
      that.  But if it does not implement it, we'll try our best guess.
  
      If there are no users returned by msg2usernames, then None is returned.