这样的元素的DTD该怎么写：可以有子元素，不能有值？

比如这样的合法：
<Elem1 prop1="prob1">
  <SubElem1 prob2="prob2" />
  <SubElem2 prob2="prob2" />
</Elem1>

而这样的非法：
<Elem1 prop1="prob1">
  <SubElem1 prob2="prob2" />
  一段文字
  <SubElem2 prob2="prob2" />
</Elem1>



谢谢！

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE Elem1[
<!ELEMENT Elem1 (SubElem1,SubElem2)>
<!ELEMENT SubElem1 EMPTY>
<!ELEMENT SubElem2 EMPTY>
<!ATTLIST Elem1 prop1 CDATA #REQUIRED>
<!ATTLIST SubElem1 prob2 CDATA #REQUIRED>
<!ATTLIST SubElem2 prob2 CDATA #REQUIRED>
]>
<Elem1 prop1="prob1">
  <SubElem1 prob2="prob2" />
  <SubElem2 prob2="prob2" />
</Elem1>