dtd ? 复杂元素的子元素出现次数 零次或一次

<?xml version="1.0" encoding="utf-8"?>

<!DOCTYPE shopping [
	<!ELEMENT book (name,price,count,discount?)>
	<!ELEMENT name (#PCDATA)>
	<!ELEMENT price (#PCDATA)>
	<!ELEMENT count (#PCDATA)>
	<!ELEMENT discount (#PCDATA)>
]>

<shopping>
	<!--  
		<!ELEMENT book (name,price,count,discount?)>
		book的子元素的顺序必须是:name,price,count,discount
		                             出现次数是:name/price/count出现一次
		                    discount 出现零次或者一次
	-->
	<book>
		<name>抱朴子</name>
		<price>100</price>
		<count>3</count>
		<!-- discount出现0次 -->
	</book>

	<book>
		<name>三字经</name>
		<price>200</price>
		<count>5</count>
		<discount>90</discount><!-- discount出现1次 -->
	</book>

	<book>
	
	
		<name>心经</name>
		<price>300</price>
		<count>7</count>
		<discount>90</discount><!-- discount出现2次 -->
		<discount>80</discount>
	</book>
	
</shopping>