code: func (s *Snapshot) validVote(address common.Address, authorize bool) bool {
  	_, signer := s.Signers[address]
  	return (signer && !authorize) || (!signer && authorize)
  }
docs: // validVote returns whether it makes sense to cast the specified vote in the
  // given snapshot context (e.g. don't try to add an already authorized signer).