<!ELEMENT grammar (ref)+>
<!ELEMENT ref (p)+>
<!ATTLIST ref id ID #REQUIRED>
<!ELEMENT xref EMPTY>
<!ATTLIST xref id IDREF #REQUIRED>
<!ELEMENT choice (p)+>
<!ELEMENT p (#PCDATA|p|xref|choice)*>
<!ATTLIST p class CDATA #IMPLIED>
<!ATTLIST p chance CDATA #IMPLIED>