* [CODE(perl)@en[undef]] (Perl)

@@ ・・・

* Null 値 (XML Schema パターン)

[1] [CITE@en-US[Basic XML Schema Patterns for Databinding Version 1.0]] ([TIME[2009-05-05 01:07:15 +09:00]] 版) <http://www.w3.org/TR/2009/NOTE-xmlschema-patterns-20090505/#optional>