WordPress : Register Custom Taxonomy Non-hierarchical

gistfile1

// Add new taxonomy, NOT hierarchical (like tags)
  $labels = array(
    'name'                         => _x( 'Writers', 'taxonomy general name' ),
    'singular_name'                => _x( 'Writer', 'taxonomy singular name' ),
    'search_items'                 => __( 'Search Writers' ),
    'popular_items'                => __( 'Popular Writers' ),
    'all_items'                    => __( 'All Writers' ),
    'parent_item'                  => null,
    'parent_item_colon'            => null,
    'edit_item'                    => __( 'Edit Writer' ), 
    'update_item'                  => __( 'Update Writer' ),
    'add_new_item'                 => __( 'Add New Writer' ),
    'new_item_name'                => __( 'New Writer Name' ),
    'separate_items_with_commas'   => __( 'Separate writers with commas' ),
    'add_or_remove_items'          => __( 'Add or remove writers' ),
    'choose_from_most_used'        => __( 'Choose from the most used writers' ),
    'not_found'                    => __( 'No writers found.' ),
    'menu_name'                    => __( 'Writers' )
  ); 

  $args = array(
    'hierarchical'            => false,
    'labels'                  => $labels,
    'show_ui'                 => true,
    'show_admin_column'       => true,
    'update_count_callback'   => '_update_post_term_count',
    'query_var'               => true,
    'rewrite'                 => array( 'slug' => 'writer' )
  );

  register_taxonomy( 'writer', 'book', $args );
}