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Wordpress Rewrite rule for extra query variable in category view
function my_rewrite_rules($rules)
{
    global $wp_rewrite;
    $my_rule = array(
        '^boxes/category/(.+)/' => '^category/' . $matches[1] . '?cp-type=boxes'
    );
    return array_merge($my_rule, $rules);
}
add_filter('page_rewrite_rules', 'my_rewrite_rules');

function my_query_vars($vars)
{
    $my_vars = array(
        'cp-type'
    );
    return array_merge($my_vars, $vars);
}
add_filter('query_vars', 'my_query_vars');