[1] [CITE@EN[W3C XML Schema Definition Language (XSD) 1.1 Part 2: Datatypes]]
( ([TIME[2012-04-04 21:34:51 +09:00]] 版))
<http://www.w3.org/TR/xmlschema11-2/#nonNegativeInteger>

[2] [CITE@EN[XML Schema Part 2: Datatypes Second Edition]]
( ([TIME[2004-10-28 21:44:54 +09:00]] 版))
<http://www.w3.org/TR/xmlschema-2/#nonNegativeInteger>