docs: public HeadedSyntacticCategory getArgumentType() {
      SyntacticCategory argumentSyntax = syntacticCategory.getArgument();
      int[] argumentSemantics = ArrayUtils.copyOfRange(semanticVariables, rootIndex + 1, semanticVariables.length);
      int argumentRoot = argumentSyntax.getNumReturnSubcategories();
      return new HeadedSyntacticCategory(argumentSyntax, argumentSemantics, argumentRoot);
    }