Linux puskom-ProLiant-DL385-Gen10 5.4.0-150-generic #167~18.04.1-Ubuntu SMP Wed May 24 00:51:42 UTC 2023 x86_64

//usr/share/doc/libxml-libxml-perl/examples/dtd.xml

 <!DOCTYPE doc [
<!ELEMENT doc (#PCDATA)>
<!ENTITY foo " test ">
]>
<doc>This is a valid document &foo; !</doc>