less []string
	op   string // Either "<" or "!<"
	def  []string
}

type syntaxError string

func (e syntaxError) Error() string {
	return string(e)
}

// parseRules parses the rules of a DAG.
func parseRules(rules string) (out []rule, err error) {
	defer func() {
		e := recover()
		switch e := e.(type) {
		case nil:
			return
		case syntaxError:
			err = e
		default:
			panic(e)