</para>
 </sect2>

 <sect2 id="isn-funcs-ops">
  <title>Functions and Operators</title>

  <para>
   The <filename>isn</filename> module provides the standard comparison operators,
   plus B-tree and hash indexing support for all these data types.  In
   addition, there are several specialized functions, shown in <xref linkend="isn-functions"/>.
   In this table,
   <type>isn</type> means any one of the module's data types.
  </para>

  <table id="isn-functions">
   <title><filename>isn</filename> Functions</title>
    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Function
       </para>
       <para>
        Description
       </para></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm><primary>make_valid</primary></indexterm>
        <function>make_valid</function> ( <type>isn</type> )
        <returnvalue>isn</returnvalue>
       </para>
       <para>
        Clears the invalid-check-digit flag of the value.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm><primary>is_valid</primary></indexterm>
        <function>is_valid</function> ( <type>isn</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Checks for the presence of the invalid-check-digit flag.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <indexterm><primary>isn_weak</primary></indexterm>
        <function>isn_weak</function> ( <type>boolean</type> )
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Sets the weak input mode, and returns the new setting.
        This function is retained for backward compatibility.
        The recommended way to set weak mode is via
        the <varname>isn.weak</varname> configuration parameter.
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <function>isn_weak</function> ()
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Returns the current status of the weak mode.
        This function is retained for backward compatibility.
        The recommended way to check weak mode is via
        the <varname>isn.weak</varname> configuration parameter.
       </para></entry>
      </row>
     </tbody>
    </tgroup>
  </table>
 </sect2>

 <sect2 id="isn-configuration-parameters">
  <title>Configuration Parameters</title>

  <variablelist>
   <varlistentry id="isn-configuration-parameters-weak">
    <term>
     <varname>isn.weak</varname> (<type>boolean</type>)
     <indexterm>
      <primary><varname>isn.weak</varname> configuration parameter</primary>
     </indexterm>
    </term>
    <listitem>
     <para>
      <varname>isn.weak</varname> enables the weak input mode, which allows
      ISN input values to be accepted even when their check digit is wrong.
      The default is <literal>false</literal>, which rejects invalid check
      digits.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   Why would you want to use the weak mode? Well, it could be that
   you have a huge collection of ISBN numbers, and that there are so many of
   them that for weird reasons some have the wrong check digit (perhaps the
   numbers were scanned from a printed list and the OCR got the numbers wrong,
   perhaps the numbers were manually captured... who knows). Anyway, the point
   is you might want to clean the mess up, but you still want to be able to
   have all the numbers in your database and maybe use an external tool to
   locate the invalid numbers in the database so you can verify the
   information and validate