code: public static String underscore(String camelCaseString) {
          String[] words = splitByCharacterTypeCamelCase(camelCaseString);
          return TextUtils.join("_", words).toLowerCase();
      }
docs: Turns a camel case string into an underscored one, e.g. "HelloWorld"
  becomes "hello_world".
  
  @param camelCaseString the string to underscore
  @return the underscored string