docs: public String singularize( Object word ) {
          if (word == null) return null;
          String wordStr = word.toString().trim();
          if (wordStr.length() == 0) return wordStr;
          if (isUncountable(wordStr)) return wordStr;
          for (Rule rule : this.singulars) {
              String result = rule.apply(wordStr);
              if (result != null) return result;
          }
          return wordStr;
      }