4. References

1. RFC 2119: Key words for use in RFCs to Indicate Requirement Levels https://www.ietf.org/rfc/rfc2119.txt

2. XML Information Set (Second Edition) https://www.w3.org/TR/2004/REC-xml-infoset-20040204/

3. Namespaces in XML 1.1 (Second Edition) https://www.w3.org/TR/xml-names11/

4. Unicode® 13.0.0 https://www.unicode.org/versions/Unicode13.0.0/

5. Extensible Markup Language (XML) 1.1 (Second Edition) https://www.w3.org/TR/xml11/

6. XML Linking Language (XLink) Version 1.1 https://www.w3.org/TR/xlink11/

7. Mathematical Markup Language (MathML) Version 2.0 (Second Edition) https://www.w3.org/TR/MathML2/