/usr/share/xml/dia/sheet.dtd is in dia-common 0.97.3+git20160930-8.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 | <!ELEMENT sheet (name, description*, contents) >
<!ATTLIST sheet
xmlns CDATA #FIXED 'http://www.lysator.liu.se/~alla/dia/dia-sheet-ns' >
<!ELEMENT name (#PCDATA)* >
<!ELEMENT description (#PCDATA)* >
<!ATTLIST description
xml:lang NMTOKEN #IMPLIED 'C' >
<!ELEMENT alias (#PCDATA)* >
<!ATTLIST alias
name CDATA #REQUIRED >
<!ELEMENT contents (object | shape | br)* >
<!ELEMENT object (description | alias)* >
<!ATTLIST object
name CDATA #REQUIRED
intdata CDATA #IMPLIED >
<!ELEMENT shape (description)* >
<!ATTLIST shape
name CDATA #REQUIRED >
<!ELEMENT br EMPTY>
|