How to use an Element child as an attribute in another element within an XML DTD?

[Uswichlmy]

I am almost finished creating a DTD for my XML file but I am not sure how to complete the last part. Here is the XML file:\[code\]<?xml version="1.0" encoding="ISO-8859-1"?><!DOCTYPE library SYSTEM "library.dtd"><library><authors> <author aid="a1">Bill Evjen</author> <author aid="a2">Michael Kay</author> <author aid="a3">Kevin Goldberg</author> <author aid="a4">Michael Morrison</author></authors><books><book bookID="b001" author="a2"> <title>XSLT 2.0 and XPath 2.0 Programmer's Reference</title> <stock>4</stock> <publisher>John Wiley</publisher> <year>2009</year> <use type="advanced" /> <use type="reference" /></book><book bookID="b002" author="a1 a2"> <title>Professional XML (Programmer to Programmer) </title> <stock>2</stock> <publisher>John Wiley</publisher> <year>2007</year> <use type="professional" /> <use type="advanced" /> <use type="reference" /></book><book bookID="b003" author="a3"> <title>XML: Visual QuickStart Guide</title> <stock>3</stock> <publisher>Peachpit Press</publisher> <year>2008</year> <use type="introductory" /> <use type="reference" /></book><book bookID="b004" author="a4"> <title>Sams Teach Yourself XML in 24 Hours</title> <stock>5</stock> <publisher>SAMS</publisher> <year>2005</year></book></books></library>\[/code\]Here is my DTD, I have commented the line I cannot figure out:\[code\]<!-- DTD for the scot ski example --><!ELEMENT library (authors | books)*><!ELEMENT authors (author)*><!ELEMENT author (#PCDATA) ><!ATTLIST author aid CDATA #REQUIRED><!ELEMENT books (book)*><!ELEMENT book (title, stock, publisher, year, use*)><!ATTLIST book bookID ID #REQUIRED><!ATTLIST book author (authors) #REQUIRED ><!-- How to do this correctly --><!ELEMENT title (#PCDATA) ><!ELEMENT stock (#PCDATA) ><!ELEMENT publisher (#PCDATA) ><!ELEMENT year (#PCDATA) ><!ELEMENT use (#PCDATA) ><!ATTLIST use type ( advanced | reference | introductory | professional) #REQUIRED >\[/code\]I have experimented for a while and tried to learn from online examples, but I am stuck, I cannot see how to do this correctly.