public function loadNodesFromProprefs(array $proprefs) {
    $loadedNodes = [];

    foreach ($proprefs as $propref) {
      $nodes = $this->loadNodesFromPropref($propref) ?: [];

      if (count($nodes) > 0) {
        foreach ($nodes as $node) {
          $loadedNodes[$propref][] = $node;
        }
      }
    }

    return $loadedNodes;
  }