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225 lines
7.3 KiB
XML
225 lines
7.3 KiB
XML
<?xml version="1.0" encoding="iso-8859-1"?>
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<!-- $Revision: 1.14 $ -->
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<refentry id="function.sqlite-create-aggregate">
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<refnamediv>
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<refname>sqlite_create_aggregate</refname>
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<refname>SQLiteDatabase->createAggregate</refname>
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<refpurpose>Register an aggregating UDF for use in SQL statements</refpurpose>
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</refnamediv>
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<refsect1 role="description">
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&reftitle.description;
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<methodsynopsis>
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<type>void</type><methodname>sqlite_create_aggregate</methodname>
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<methodparam><type>resource</type><parameter>dbhandle</parameter></methodparam>
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<methodparam><type>string</type><parameter>function_name</parameter></methodparam>
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<methodparam><type>callback</type><parameter>step_func</parameter></methodparam>
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<methodparam><type>callback</type><parameter>finalize_func</parameter></methodparam>
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<methodparam choice="opt"><type>int</type><parameter>num_args</parameter></methodparam>
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</methodsynopsis>
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<para>Object oriented style (method):</para>
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<classsynopsis>
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<ooclass><classname>SQLiteDatabase</classname></ooclass>
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<methodsynopsis>
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<type>void</type><methodname>createAggregate</methodname>
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<methodparam><type>string</type><parameter>function_name</parameter></methodparam>
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<methodparam><type>callback</type><parameter>step_func</parameter></methodparam>
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<methodparam><type>callback</type><parameter>finalize_func</parameter></methodparam>
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<methodparam choice="opt"><type>int</type><parameter>num_args</parameter></methodparam>
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</methodsynopsis>
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</classsynopsis>
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<para>
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<function>sqlite_create_aggregate</function> is similar to
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<function>sqlite_create_function</function> except that it registers
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functions that can be used to calculate a result aggregated across all the
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rows of a query.
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</para>
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<para>
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The key difference between this function and
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<function>sqlite_create_function</function> is that two functions are
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required to manage the aggregate; <parameter>step_func</parameter> is
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called for each row of the result set. Your PHP function should
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accumulate the result and store it into the aggregation context.
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Once all the rows have been processed,
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<parameter>finalize_func</parameter> will be called and it should then
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take the data from the aggregation context and return the result.
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Callback functions should return a type understood by SQLite (i.e.
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<link linkend="language.types.intro">scalar type</link>).
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</para>
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</refsect1>
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<refsect1 role="parameters">
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&reftitle.parameters;
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<para>
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<variablelist>
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<varlistentry>
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<term><parameter>dbhandle</parameter></term>
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<listitem>
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<para>
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The SQLite Database resource; returned from <function>sqlite_open</function>
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when used procedurally. This parameter is not required
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when using the object-oriented method.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>function_name</parameter></term>
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<listitem>
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<para>
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The name of the function used in SQL statements.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>step_func</parameter></term>
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<listitem>
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<para>
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Callback function called for each row of the result set.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>finalize_func</parameter></term>
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<listitem>
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<para>
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Callback function to aggregate the "stepped" data from each row.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>num_args</parameter></term>
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<listitem>
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<para>
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Hint to the SQLite parser if the callback function accepts a
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predetermined number of arguments.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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</refsect1>
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<refsect1 role="examples">
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&reftitle.examples;
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<para>
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<example>
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<title>max_length aggregation function example</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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$data = array(
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'one',
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'two',
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'three',
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'four',
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'five',
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'six',
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'seven',
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'eight',
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'nine',
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'ten',
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);
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$dbhandle = sqlite_open(':memory:');
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sqlite_query($dbhandle, "CREATE TABLE strings(a)");
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foreach ($data as $str) {
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$str = sqlite_escape_string($str);
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sqlite_query($dbhandle, "INSERT INTO strings VALUES ('$str')");
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}
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function max_len_step(&$context, $string)
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{
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if (strlen($string) > $context) {
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$context = strlen($string);
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}
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}
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function max_len_finalize(&$context)
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{
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return $context;
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}
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sqlite_create_aggregate($dbhandle, 'max_len', 'max_len_step', 'max_len_finalize');
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var_dump(sqlite_array_query($dbhandle, 'SELECT max_len(a) from strings'));
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?>
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]]>
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</programlisting>
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</example>
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</para>
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<para>
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In this example, we are creating an aggregating function that will
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calculate the length of the longest string in one of the columns of the
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table. For each row, the <literal>max_len_step</literal> function is
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called and passed a <parameter>context</parameter> parameter. The context
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parameter is just like any other PHP variable and be set to hold an array
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or even an object value. In this example, we are simply using it to hold
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the maximum length we have seen so far; if the
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<parameter>string</parameter> has a length longer than the current
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maximum, we update the context to hold this new maximum length.
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</para>
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<para>
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After all of the rows have been processed, SQLite calls the
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<literal>max_len_finalize</literal> function to determine the aggregate
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result. Here, we could perform some kind of calculation based on the
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data found in the <parameter>context</parameter>. In our simple example
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though, we have been calculating the result as the query progressed, so we
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simply need to return the context value.
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</para>
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<note>
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<para>
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The example above will not work correctly if the column contains binary
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data. Take a look at the manual page for
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<function>sqlite_udf_decode_binary</function> for an explanation of why
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this is so, and an example of how to make it respect the binary encoding.
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</para>
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</note>
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<tip>
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<para>
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It is NOT recommended for you to store a copy of the values in the context
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and then process them at the end, as you would cause SQLite to use a lot of
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memory to process the query - just think of how much memory you would need
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if a million rows were stored in memory, each containing a string 32 bytes
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in length.
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</para>
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</tip>
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<tip>
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<para>
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You can use <function>sqlite_create_function</function> and
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<function>sqlite_create_aggregate</function> to override SQLite native
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SQL functions.
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</para>
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</tip>
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</refsect1>
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<refsect1 role="seealso">
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&reftitle.seealso;
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<para>
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<simplelist>
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<member><function>sqlite_create_function</function></member>
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<member><function>sqlite_udf_encode_binary</function></member>
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<member><function>sqlite_udf_decode_binary</function></member>
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</simplelist>
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</para>
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</refsect1>
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</refentry>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode: sgml
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sgml-omittag:t
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sgml-shorttag:t
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sgml-minimize-attributes:nil
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sgml-indent-data:t
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sgml-parent-document:nil
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sgml-default-dtd-file:"../../manual.ced"
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sgml-exposed-tags:nil
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sgml-local-catalogs:nil
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sgml-local-ecat-files:nil
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End:
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vim600: syn=xml fen fdm=syntax fdl=2 si
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vim: et tw=78 syn=sgml
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vi: ts=1 sw=1
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-->
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