my $self = shift;
    $self->run_modes(

        # default
        'index' => 'index',

        # user's tag cloud
        'tag_cloud' => 'tag_cloud',
        'edit_tags' => 'edit_tags',
        'tag_find'  => 'tag_find',

        ..
        # called on unknown mode.
        'AUTOLOAD' => 'unknown_mode',
    );


##</code><code>##

sub unknown_mode
{
    my ( $self, $requested ) = (@_);
    my $q        = $self->query();
    my $session  = $self->param('session');
    my $username = $session->param('logged_in');

    $requested = HTML::Entities::encode_entities($requested);

    if ( defined($username) && length($username) )
    {
        return "<p>unknown mode '$requested' for logged in user $username</p>";
    }
    else
    {
        return "<p>Unknown mode '$requested' for anonymous user.</p>";
    }
}