Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC 3986]
Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI
10.17487/RFC3986, January 2005, <http://www.rfc-editor.org/info/rfc3986>.
[RFC 5646]
Phillips, A., Ed., and M. Davis, Ed., "Tags for
Identifying Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646, September 2009,
<http://www.rfc-editor.org/info/rfc5646>.