$aXML =~ s@&lab;@\<@gs;
   $aXML =~ s@&rab;@\>@gs;
   $aXML =~ s@&lcb;@\(@gs;
   $aXML =~ s@&rcb;@\)@gs;
   $aXML =~ s@&lsb;@\[@gs;
   $aXML =~ s@&rsb;@\]@gs;

##</code><code>##


pseudocode
----------

$aXML =~ s@`@<backtick>@gs;

while ( commands remain unprocessed )
   {
    foreach match for <any_command>
       {
        substitute for <`any_command>
        set found_command boolean flag true
       }

    if (found_command)
      {
       while ( find match for <`command>data</command> ) 
            {
             process if (
                         no nested ` chars found             
                        )

            }
      }
   }


##</code><code>##


  <baz><qd>foo</qd></baz>


##</code><code>##


  <baz>bar</baz>