AW: XML Schema -> DTD?

<quotation from="[1]">
Conformance
The XML schema language must:
...
4.define the relationship between schemas and XML DTDs, and their
information sets;
</quotation>

What is the meaning of "information set"? 

> Unless I'm missing something, it so happens that the
> particular schema you give does correspond to a simple DTD, 

The problem in this DTD is the definition of element "myTag":

<!ELEMENT myTag EMPTY>
<!ATTLIST myTag key (IDREF|ID) #REQUIRED>	 <!-- (IDREF|ID) not
valid! -->