def _assign_token_ids(self, id_factory):
for token in self.tokens_to_add():<EOL><INDENT>token._give_id(id_factory)<EOL><DEDENT>
Assign id numbers to any tokens that will be added to the world by this 
message.

This method is called by Actor but not by ServerActor, so it's 
guaranteed to be called exactly once.  In fact, this method is not 
really different from the constructor, except that the id_factory 
object is nicely provided.  That's useful for assigning ids to tokens 
but probably nothing else.  This method is called before _check() so 
that _check() can make sure that valid ids were assigned (although by 
default it doesn't).