Atom配信フォーマット（Atomの拡張）

   This specification describes Atom's XML markup vocabulary.  Markup
   from other vocabularies ("foreign markup") can be used in an Atom
   Document.  Note that the atom:content element is designed to support
   the inclusion of arbitrary foreign markup.

   The Atom namespace is reserved for future forward-compatible
   revisions of Atom.  Future versions of this specification could add
   new elements and attributes to the Atom markup vocabulary.  Software
   written to conform to this version of the specification will not be
   able to process such markup correctly and, in fact, will not be able
   to distinguish it from markup error.  For the purposes of this
   discussion, unrecognized markup from the Atom vocabulary will be
   considered "foreign markup".

   Atom Processors that encounter foreign markup in a location that is
   legal according to this specification MUST NOT stop processing or
   signal an error.  It might be the case that the Atom Processor is
   able to process the foreign markup correctly and does so.  Otherwise,
   such markup is termed "unknown foreign markup".

   When unknown foreign markup is encountered as a child of atom:entry,
   atom:feed, or a Person construct, Atom Processors MAY bypass the
   markup and any textual content and MUST NOT change their behavior as
   a result of the markup's presence.

   When unknown foreign markup is encountered in a Text Construct or
   atom:content element, software SHOULD ignore the markup and process
   any text content of foreign elements as though the surrounding markup
   were not present.

   Atom allows foreign markup anywhere in an Atom document, except where
   it is explicitly forbidden.  Child elements of atom:entry, atom:feed,
   atom:source, and Person constructs are considered Metadata elements
   and are described below.  Child elements of Person constructs are
   considered to apply to the construct.  The role of other foreign
   markup is undefined by this specification.

   A Simple Extension element MUST NOT have any attributes or child
   elements.  The element MAY contain character data or be empty.
   Simple Extension elements are not Language-Sensitive.

   simpleExtensionElement =
      element * - atom:* {
         text
      }

   The element can be interpreted as a simple property (or name/value
   pair) of the parent element that encloses it.  The pair consisting of
   the namespace-URI of the element and the local name of the element
   can be interpreted as the name of the property.  The character data
   content of the element can be interpreted as the value of the
   property.  If the element is empty, then the property value can be
   interpreted as an empty string.

   The root element of a Structured Extension element MUST have at least
   one attribute or child element.  It MAY have attributes, it MAY
   contain well-formed XML content (including character data), or it MAY
   be empty.  Structured Extension elements are Language-Sensitive.

   structuredExtensionElement =
      element * - atom:* {
         (attribute * { text }+,
            (text|anyElement)*)
       | (attribute * { text }*,
          (text?, anyElement+, (text|anyElement)*))
      }

   The structure of a Structured Extension element, including the order
   of its child elements, could be significant.

   This specification does not provide an interpretation of a Structured
   Extension element.  The syntax of the XML contained in the element
   (and an interpretation of how the element relates to its containing
   element) is defined by the specification of the Atom extension.