code: public String findPrefix(String uri){
          if(uri==null)
              uri = "";
          String prefix = getPrefix(uri);
          if(prefix==null){
              String defaultURI = getURI("");
              if(defaultURI==null)
                  defaultURI = "";
              if(Util.equals(uri, defaultURI))
                  prefix = "";
          }
          return prefix;
      }
docs: Return one of the prefixes mapped to a Namespace URI.
  
  <p>If more than one prefix is currently mapped to the same
  URI, this method will make an arbitrary selection;
  
  <p>Unlike {@link #getPrefix(String)} this method, this returns empty
  prefix if the given uri is bound to default prefix.