code: public static String suggestPrefix(String uri){
          if(uri==null)
              uri = "";
          return suggested.getProperty(uri);
      }
docs: Returns suggested prefix fore the given {@code uri}.
  
  @param uri namespace uri
  @return empty string if {@code uri} is null or empty.
  If the {@code uri} is one of the constants
  defined in this class, then it returns commonly
  used prefix. Otherwise it returns null.