lexer/lexer.go
func Lex(xpath string) chan XItem {
	l := &Lexer{
		input: xpath,
		items: make(chan XItem),
	}
	go l.run()
	return l.items
}
func Lex(xpath string) chan XItem {
	l := &Lexer{
		input: xpath,
		items: make(chan XItem),
	}
	go l.run()
	return l.items
}
//Lex an XPath expresion on the io.Reader