<!DOCTYPE a [

        <!ELEMENT a (b)* >

        <!ELEMENT  b  (c, d)>
        <!ELEMENT  c  (#PCDATA)>
        <!ATTLIST b
                x       CDATA          #REQUIRED
                y       CDATA          #IMPLIED
        >

        <!ELEMENT  d  EMPTY>
        <!ATTLIST d
                z       CDATA          #REQUIRED
        >
        ]>
<a>
  <b x="">
      <c></c>
      <d z=""/>
  </b>
</a>