code: public String getNamespaceURIByPrefix(String prefix) throws NamespaceException {
          try {
  //            int index = Integer.parseInt(prefix);
  //            return nsReg.indexToString(index);
             return nsReg.getURI(prefix);
          } catch (IllegalArgumentException e) {
              throw new NamespaceException(
                      "Unknown namespace prefix: " + prefix, e);
          }
      }
docs: {@inheritDoc}