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<HTML>
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<HEAD>
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<TITLE>OpenSP - System identifiers</TITLE>
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</HEAD>
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<BODY>
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System identifiers
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There are two kinds of system identifier: formal system identifiers
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and simple system identifiers. A system identifier that does not
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start with <SAMP><</SAMP> will always be interpreted as a simple
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system identifier. A simple system identifier will always be
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interpreted either as a filename or as a URL.
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Formal system identifiers
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Formal system identifiers are based on the
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System Identifier facility defined in ISO/IEC 10744 (HyTime) Technical
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Corrigendum 1, Annex D.
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A system identifier that is a formal system
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identifier consists of a sequence of one or more storage object
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specifications. The objects specified by the storage object
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specifications are concatenated to form the entity. A storage object
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specification consists of an SGML start-tag in the reference concrete
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syntax followed by character data content. The generic identifier of
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the start-tag is the name of a storage manager. The content is a
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storage object identifier which identifies the storage object in a
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manner dependent on the storage manager. The start-tag can also
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specify attributes giving additional information about the storage
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object. Numeric character references are recognized in storage object
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identifiers and attribute value literals in the start-tag. Record
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ends are ignored in the storage object identifier as with SGML. A
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system identifier will be interpreted as a formal system identifier if
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it starts with a <SAMP><</SAMP> followed by a storage manager name,
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followed by either <SAMP>></SAMP> or white-space; otherwise it will be
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interpreted as a simple system identifier. A storage object
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identifier extends until the end of the system identifier or until the
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first occurrence of <SAMP><</SAMP> followed by a storage manager
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name, followed by either <SAMP>></SAMP> or white-space.
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The following storage managers are available:
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<SAMP>osfile</SAMP>
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The storage object identifier is a filename. If the filename is
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relative it is resolved using a base filename. Normally the base
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filename is the name of the file in which the storage object
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identifier was specified, but this can be changed using the
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<SAMP>base</SAMP> attribute. The filename will be searched for first
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in the directory of the base filename. If it is not found there, then
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it will be searched for in directories specified with the
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<SAMP>-D</SAMP> option in the order in which they were specified on
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the command line, and then in the list of directories specified by the
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environment variable <SAMP>SGML_SEARCH_PATH</SAMP>. The list
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is separated by colons under Unix and by semi-colons under MSDOS.
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<SAMP>osfd</SAMP>
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The storage object identifier is an integer specifying a file
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descriptor. Thus a system identifier of <SAMP><osfd>0</SAMP> will
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refer to the standard input (STDIN), 1 to standard output
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(STDOUT), and 2 to standard error
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(STDERR). Arbitrary file descriptors may be used so you
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could say <samp><osfd>3</samp> to get an extra input or output
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stream independant of the usual three. This is useful, e.g., for
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generating catalog entries or SGML declarations on the fly. You can
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use an osfd storage object anywhere you can use a sysid, including on
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the command line.
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<SAMP>url</SAMP>
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The storage object identifier is a URL. Only the <SAMP>http</SAMP>
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scheme is currently supported and not on all systems.
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<SAMP>neutral</SAMP>
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The storage manager is the storage manager of storage object in which
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the system identifier was specified (the underlying storage
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manager). However if the underlying storage manager does not
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support named storage objects (ie it is <SAMP>osfd</SAMP>), then the
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storage manager will be <SAMP>osfile</SAMP>. The storage object
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identifier is treated as a relative, hierarchical name separated by
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slashes (<SAMP>/</SAMP>) and will be transformed as appropriate for
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the underlying storage manager.
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<SAMP>literal</SAMP>
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The bit combinations of the storage object identifier are
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the contents of the storage object.
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The following attributes are supported:
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<SAMP>records</SAMP>
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This describes how records are delimited in the storage object:
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<SAMP>cr</SAMP>
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Records are terminated by a carriage return.
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<SAMP>lf</SAMP>
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Records are terminated by a line feed.
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<SAMP>crlf</SAMP>
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Records are terminated by a carriage return followed by a line feed.
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<SAMP>find</SAMP>
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Records are terminated by whichever of
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<SAMP>cr</SAMP>,
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<SAMP>lf</SAMP>
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or
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<SAMP>crlf</SAMP>
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is first encountered in the storage object.
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<SAMP>asis</SAMP>
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No recognition of records is performed.
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The default is <SAMP>find</SAMP> except for NDATA entities for which
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the default is <SAMP>asis</SAMP>. This attribute is not applicable to
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the <SAMP>literal</SAMP> storage manager.
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When records are recognized in a storage object, a record start is
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inserted at the beginning of each record, and a record end at the end
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of each record. If there is a partial record (a record that doesn't
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end with the record terminator) at the end of the entity, then a
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record start will be inserted before it but no record end will be
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inserted after it.
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The attribute name and <SAMP>=</SAMP> can be omitted for this attribute.
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<SAMP>zapeof</SAMP>
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This specifies whether a Control-Z character that occurs as the final byte
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in the storage object should be stripped.
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The following values are allowed:
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<SAMP>zapeof</SAMP>
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A final Control-Z should be stripped.
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<SAMP>nozapeof</SAMP>
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A final Control-Z should not be stripped.
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The default is <SAMP>zapeof</SAMP> except for NDATA entities, entities
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declared in storage objects with <SAMP>zapeof=nozapeof</SAMP> and
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storage objects with <SAMP>records=asis</SAMP>. This attribute is not
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applicable to the <SAMP>literal</SAMP> storage manager.
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The attribute name and <SAMP>=</SAMP> can be omitted for this
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attribute.
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<SAMP>encoding</SAMP>
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The encoding attribute specifies the encoding of the storage object.
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This attribute is used when the encoding is independent of the
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document character set.
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The value must be the name of an encoding.
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This attribute is not applicable to the
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<SAMP>literal</SAMP> storage manager.
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<SAMP>bctf</SAMP>
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The BCTF attribute specifies that the encoding of the storage
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object.
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This attribute is used when the encoding is
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document character set dependent.
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The value must be the name of a BCTF.
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This attribute is not applicable to the
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<SAMP>literal</SAMP> storage manager.
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<SAMP>tracking</SAMP>
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This specifies whether line boundaries should be tracked for this
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object: a value of <SAMP>track</SAMP> specifies that they should; a
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value of <SAMP>notrack</SAMP> specifies that they should not. The
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default value is <SAMP>track</SAMP>. Keeping track of where line
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boundaries occur in a storage object requires approximately one byte
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of storage per line and it may be desirable to disable this for very
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large storage objects.
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The attribute name and
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<SAMP>=</SAMP>
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can be omitted for this attribute.
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<SAMP>base</SAMP>
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When the storage object identifier specified in the content of the
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storage object specification is relative, this specifies the base
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storage object identifier relative to which that storage object
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identifier should be resolved.
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When not specified a storage object identifier is interpreted
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relative to the storage object in which it is specified,
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provided that this has the same storage manager.
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This applies both to system identifiers specified in SGML
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documents and to system identifiers specified in the catalog entry
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files.
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<SAMP>smcrd</SAMP>
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The value is a single character that will be recognized in storage
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object identifiers (both in the content of storage object
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specifications and in the value of <SAMP>base</SAMP> attributes) as a
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storage manager character reference delimiter when followed by a
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digit. A storage manager character reference is like an SGML numeric
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character reference except that the number is interpreted as a
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character number in the inherent character set of the storage manager
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rather than the document character set. The default is for no
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character to be recognized as a storage manager character reference
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delimiter. Numeric character references cannot be used to prevent
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recognition of storage manager character reference delimiters.
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<SAMP>fold</SAMP>
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This applies only to the <SAMP>neutral</SAMP> storage manager. It
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specifies whether the storage object identifier should be folded to
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the customary case of the underlying storage manager if storage object
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identifiers for the underlying storage manager are case sensitive.
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The following values are allowed:
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<SAMP>fold</SAMP>
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The storage object identifier will be folded.
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<SAMP>nofold</SAMP>
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The storage object identifier will not be folded.
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The default value is <SAMP>fold</SAMP>. The attribute name and
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<SAMP>=</SAMP> can be omitted for this attribute.
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For example, on Unix filenames are case-sensitive and the customary
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case is lower-case. So if the underlying storage manager were
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<SAMP>osfile</SAMP> and the system was a Unix system, then
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<SAMP><neutral>FOO.SGM</SAMP> would be equivalent to
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<SAMP><osfile>foo.sgm</SAMP>.
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Simple system identfiers
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A simple system identifier is interpreted as a storage object
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identifier with a storage manager that depends on where the system
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identifier was specified: if it was specified in a storage object
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whose storage manager was <SAMP>url</SAMP> or if the system identifier
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looks like an absolute URL in a supported scheme, the storage manager
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will be <SAMP>url</SAMP>; otherwise the storage manager will be
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<SAMP>osfile</SAMP>. The storage manager attributes are defaulted as
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for a formal system identifier. Numeric character references are not
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recognized in simple system identifiers.
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</BODY>
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</HTML>
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