function callback( $path = '', $blog_id = 0, $taxonomy = 'category', $slug = 0 ) {
    $slug = urldecode( $slug );
    $blog_id = $this->api->switch_to_blog_and_validate_user( $this->api->get_blog_id( $blog_id ) );
    if ( is_wp_error( $blog_id ) ) {
      return $blog_id;
    }

    if ( defined( 'IS_WPCOM' ) && IS_WPCOM ) {
      $this->load_theme_functions();
    }

    $user = wp_get_current_user();
    if ( ! $user || is_wp_error( $user ) || ! $user->ID ) {
      return new WP_Error( 'authorization_required', 'An active access token must be used to manage taxonomies.', 403 );
    }

    $taxonomy_meta = get_taxonomy( $taxonomy );
    if ( false === $taxonomy_meta || (
        ! $taxonomy_meta->public &&
        ! current_user_can( $taxonomy_meta->cap->manage_terms ) &&
        ! current_user_can( $taxonomy_meta->cap->edit_terms ) &&
        ! current_user_can( $taxonomy_meta->cap->delete_terms ) ) ) {
      return new WP_Error( 'invalid_taxonomy', 'The taxonomy does not exist', 400 );
    }

    if ( $this->api->ends_with( $path, '/delete' ) ) {
      return $this->delete_term( $path, $blog_id, $slug, $taxonomy );
    } else if ( $this->api->ends_with( $path, '/new' ) ) {
      return $this->new_term( $path, $blog_id, $taxonomy );
    }

    return $this->update_term( $path, $blog_id, $slug, $taxonomy );
  }