Godeps/_workspace/src/gopkg.in/mgo.v2/txn/txn.go
func tokenFor(t *transaction) token {
	nonce := t.Nonce
	if nonce == "" {
		nonce = newNonce()
	}
	return token(t.Id.Hex() + "_" + nonce)
}
func tokenFor(t *transaction) token {
	nonce := t.Nonce
	if nonce == "" {
		nonce = newNonce()
	}
	return token(t.Id.Hex() + "_" + nonce)
}
// 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.