code: protected function requireSingleChild(\SimpleXMLElement $html_table, $tag_name)
      {
          if ( ! $child = $html_table->$tag_name) {
              throw new \InvalidArgumentException(
                  'No <'.$tag_name.'> found in <'.$html_table->getName().'>'
              );
          }
  
          /** @var \SimpleXmlElement $child */
  
          if ($child->count() > 1) {
              throw new \InvalidArgumentException(
                  'Multiple <'.$tag_name.'> found in <'.$html_table->getName().'>'
              );
          }
  
          return $child;
      }
docs: @param \SimpleXMLElement $html_table
  @param string            $tag_name
  
  @return \SimpleXMLElement