docs: public static int difference(String s1, String s2) {
  
          /** @todo: check if this is the standard algorithm */
          if ((s1 == null) || (s2 == null)) {
              return 0;
          }
  
          s1 = soundex(s1);
          s2 = soundex(s2);
  
          int e = 0;
  
          for (int i = 0; i < 4; i++) {
              if (s1.charAt(i) != s2.charAt(i)) {
                  e++;
              }
          }
  
          return e;
      }