Atom配信フォーマット（Atom文書）

   This specification describes two kinds of Atom Documents: Atom Feed
   Documents and Atom Entry Documents.

   An Atom Feed Document is a representation of an Atom feed, including
   metadata about the feed, and some or all of the entries associated
   with it.  Its root is the atom:feed element.

   An Atom Entry Document represents exactly one Atom entry, outside of
   the context of an Atom feed.  Its root is the atom:entry element.

   namespace atom = "http://www.w3.org/2005/Atom"
   start = atomFeed | atomEntry

   Both kinds of Atom Documents are specified in terms of the XML
   Information Set, serialized as XML 1.0 [W3C.REC-xml-20040204] and
   identified with the "application/atom+xml" media type.  Atom
   Documents MUST be well-formed XML.  This specification does not
   define a DTD for Atom Documents, and hence does not require them to
   be valid (in the sense used by XML).

   Atom allows the use of IRIs [RFC3987].  Every URI [RFC3986] is also
   an IRI, so a URI may be used wherever below an IRI is named.  There
   are two special considerations: (1) when an IRI that is not also a
   URI is given for dereferencing, it MUST be mapped to a URI using the
   steps in Section 3.1 of [RFC3987] and (2) when an IRI is serving as
   an atom:id value, it MUST NOT be so mapped, so that the comparison
   works as described in Section 4.2.6.1.

   Any element defined by this specification MAY have an xml:base
   attribute [W3C.REC-xmlbase-20010627].  When xml:base is used in an
   Atom Document, it serves the function described in section 5.1.1 of
   [RFC3986], establishing the base URI (or IRI) for resolving any
   relative references found within the effective scope of the xml:base
   attribute.

   Any element defined by this specification MAY have an xml:lang
   attribute, whose content indicates the natural language for the
   element and its descendents.  The language context is only
   significant for elements and attributes declared to be "Language-
   Sensitive" by this specification.  Requirements regarding the content
   and interpretation of xml:lang are specified in XML 1.0
   [W3C.REC-xml-20040204], Section 2.12.

   atomCommonAttributes =
      attribute xml:base { atomUri }?,
      attribute xml:lang { atomLanguageTag }?,
      undefinedAttribute*

   Atom is an extensible format.  See Section 6 of this document for a
   full description of how Atom Documents can be extended.

   Atom Processors MAY keep state sourced from Atom Feed Documents and
   combine them with other Atom Feed Documents, in order to facilitate a
   contiguous view of the contents of a feed.  The manner in which Atom
   Feed Documents are combined in order to reconstruct a feed (e.g.,
   updating entries and metadata, dealing with missing entries) is out
   of the scope of this specification.