<!-- DTD for gravitrams_nested.xml, a nested XML file -->
<!ELEMENT gravitrams (record*)>
<!ELEMENT record (machine, specifications, locations, media)>
<!ATTLIST record id CDATA #REQUIRED>
<!ELEMENT machine (machine_name, build_year, artist, built_in)>
<!ELEMENT machine_name (#PCDATA)>
<!ELEMENT build_year (#PCDATA)>
<!ELEMENT artist (#PCDATA)>
<!ELEMENT built_in (#PCDATA)>
<!ELEMENT specifications (width, length, height, number_balls, features)>
<!ELEMENT width (#PCDATA)>
<!ELEMENT length (#PCDATA)>
<!ELEMENT height (#PCDATA)>
<!ELEMENT number_balls (#PCDATA)>
<!ELEMENT features (#PCDATA)>
<!ELEMENT locations (current_location?, encounter_location, encounter_date)>
<!ELEMENT current_location (#PCDATA)>
<!ELEMENT encounter_location (#PCDATA)>
<!ELEMENT encounter_date (#PCDATA)>
<!ELEMENT media (image*, machine_url*, artist_url*)>
<!ELEMENT image EMPTY>
<!ATTLIST image src CDATA #IMPLIED>
<!ATTLIST image width CDATA #IMPLIED>
<!ATTLIST image height CDATA #IMPLIED>
<!ELEMENT url (#PCDATA)>
<!ATTLIST url type CDATA #REQUIRED>