Install docbook-ebnf by entering the following commands in the terminal:
sudo apt update sudo apt install docbook-ebnf
EBNF module for the XML version of the DocBook DTD
DocBook is a general-purpose and modular SGML and XML Document-Type Definition. This is a module that adds elements for the markup of EBNF grammar rules to DocBook. . It is fully documented in "DocBook - The Definitive Guide", available in the docbook-defguide package.