doDiff example

foreach ($elements as $tag => $restrictions) {
        // `<*>`'s attribute restrictions do not need to be compared.         if ($tag === '*') {
          continue;
        }
        $original[$tag] = $restrictions;
        $global[$tag] = $globally_allowed_attribute_restrictions;
      }

      // The subset of attribute restrictions after diffing with those on `<*>`.       $net_global_attribute_restrictions = (new self($original))
        ->doDiff(new self($global))
        ->getAllowedElements(FALSE);

      // Update each tag's attribute restrictions to the subset.       foreach ($elements as $tag => $restrictions) {
        // `<*>` remains untouched.         if ($tag === '*') {
          continue;
        }
        $this->elements[$tag] = $net_global_attribute_restrictions[$tag]
          // If the tag is absent from the subset, then its attribute           // restrictions were a strict subset of `<*>`: just allowing the tag