<?xml version='1.0' encoding='us-ascii'?> <!-- DTD for a simple "math". --> <!ELEMENT apply ( (minus, (cn | ci | apply)) | ( (minus | divide | power), (cn | ci | apply), (cn | ci | apply)) | ( (plus | times), (cn | ci | apply)*) )> <!ELEMENT minus (empty)> <!ELEMENT divide (empty)> <!ELEMENT power (empty)> <!ELEMENT plus (empty)> <!ELEMENT times (empty)> <!ELEMENT cn (#PCDATA)> <!ELEMENT ci (#PCDATA)>