<!-- XML DTD "term.dtd" -->
<!ENTITY % op "(plus|minus|div|mult|val)">
<!ENTITY % binaryop "(%op; , %op;)">

<!ELEMENT term %op;> 
<!ELEMENT plus %binaryop;> 
<!ELEMENT minus %binaryop;> 
<!ELEMENT div %binaryop;> 
<!ELEMENT mult %binaryop;> 
<!ELEMENT val (#PCDATA)>