/**
     * Outputs the HTML for the default_line_number setting to the settings page.
     */
    public function setting_default_line_number()
    {
        debug_log('function start');
        // Get the current options values.
        $options = get_option(self::SETTINGS_GROUP);
        $option_value = ArrayHelper::get_value_or_default(self::OPTION_DEFAULT_LINE_NUMBERS, $options, 0);
        debug_log('current value of ' . self::OPTION_DEFAULT_LINE_NUMBERS . ': ' . $option_value);
        $setting_name = self::SETTINGS_GROUP . '[' . self::OPTION_DEFAULT_LINE_NUMBERS . ']';
        ?>
		<input type="checkbox"
		       id="<?php 
        echo esc_attr(self::OPTION_DEFAULT_LINE_NUMBERS);
        ?>
"
		       name="<?php 
        echo esc_attr($setting_name);
        ?>
"
		       value="1" <?php 
        checked(1, $option_value, true);
        ?>
		       title="Show line numbers by default"
		/>
		<?php 
        debug_log('function end');
    }