<?xml version = "1.0" encoding = "utf-8" ?>

	<!-- planes.dtd - a document type definition for
 the planes.xml document, which specifies
 a list of used airplanes for sale -->

	<!ELEMENT planes_for_sale (ad+)>
	<!ELEMENT ad (year, make, model, color, description, 
 	 price?, seller, location)>
	<!ELEMENT year (#PCDATA)>
	<!ELEMENT make (#PCDATA)>
	<!ELEMENT model (#PCDATA)>
	<!ELEMENT color (#PCDATA)>
	<!ELEMENT description (#PCDATA)>
	<!ELEMENT price (#PCDATA)>
	<!ELEMENT seller (#PCDATA)>
	<!ELEMENT location (city, state)>
	<!ELEMENT city (#PCDATA)>
	<!ELEMENT state (#PCDATA)>

	<!ATTLIST seller phone CDATA #REQUIRED>
	<!ATTLIST seller email CDATA #IMPLIED>

	<!ENTITY c "Cessna">
	<!ENTITY p "Piper">
	<!ENTITY b "Beechcraft">