docs: public HeadedSyntacticCategory getReturnType() {
      SyntacticCategory returnSyntax = syntacticCategory.getReturn();
      int[] returnSemantics = ArrayUtils.copyOf(semanticVariables, rootIndex);
      int returnRoot = returnSyntax.getNumReturnSubcategories();
      return new HeadedSyntacticCategory(returnSyntax, returnSemantics, returnRoot);
    }