code: def persist_revision_once(tokens, revision):
      
      token_map = {id(token):token for token in tokens}
      for token in token_map.values():
          token.persist(revision)
docs: This function makes sure that a revision is only marked as persisting
      for a token once.  This is important since some diff algorithms allow
      tokens to be copied more than once in a revision.  The id(token) should
      unique to the in-memory representation of any object, so we use that as
      unique token instance identifier.