docs: public Set<HeadedSyntacticCategory> getSubcategories(Set<String> featureValues) {
      Set<HeadedSyntacticCategory> subcategories = Sets.newHashSet();
      for (SyntacticCategory newSyntax : syntacticCategory.getSubcategories(featureValues)) {
        subcategories.add(new HeadedSyntacticCategory(newSyntax, semanticVariables, rootIndex));
      }
      return subcategories;
    }