code: func tokenFor(t *transaction) token {
  	nonce := t.Nonce
  	if nonce == "" {
  		nonce = newNonce()
  	}
  	return token(t.Id.Hex() + "_" + nonce)
  }
docs: // tokenFor returns a unique transaction token that
  // is composed by t's id and a nonce. If t already has
  // a nonce assigned to it, it will be used, otherwise
  // a new nonce will be generated.