function is called by
      <command>CREATE FUNCTION</command> if it exists.  If a validator function
      is provided by the language, declare it with a command like
<synopsis>
CREATE FUNCTION <replaceable>validator_function_name</replaceable>(oid)
    RETURNS void
    AS '<replaceable>path-to-shared-object</replaceable>'
    LANGUAGE C STRICT;
</synopsis>
     </para>
    </step>

    <step performance="required" id="xplang-install-cr5">
     <para>
      Finally, the PL must be declared with the command
<synopsis>
CREATE <optional>TRUSTED</optional> LANGUAGE <replaceable>language_name</replaceable>
    HANDLER <replaceable>handler_function_name</replaceable>
    <optional>INLINE <replaceable>inline_function_name</replaceable></optional>
    <optional>VALIDATOR <replaceable>validator_function_name</replaceable></optional> ;
</synopsis>
      The optional key word <literal>TRUSTED</literal> specifies that
      the language does not grant access to data that the user would
      not otherwise have.  Trusted languages are designed for ordinary
      database users (those without superuser privilege) and allows them
      to safely create functions and
      procedures. Since PL functions are executed inside the database
      server, the <literal>TRUSTED</literal> flag should only be given
      for languages that do not allow access to database server
      internals or the file system. The languages
      <application>PL/pgSQL</application>,
      <application>PL/Tcl</application>, and
      <application>PL/Perl</application>
      are considered trusted; the languages
      <application>PL/TclU</application>,
      <application>PL/PerlU</application>, and
      <application>PL/PythonU</application>
      are designed to provide unlimited functionality and should
      <emphasis>not</emphasis> be marked trusted.
     </para>
    </step>
   </procedure>

   <para>
    <xref linkend="xplang-install-example"/> shows how the manual
    installation procedure would work with the language
    <application>PL/Perl</application>.
   </para>

   <example id="xplang-install-example">
    <title>Manual Installation