code: public void declareAlternativeNamespace(String alternativeNs, String actualNs) {
      if(actualNsToAlternative.containsKey(actualNs) || alternativeNsToActual.containsKey(alternativeNs)){
        throw new IllegalArgumentException("Cannot register two alternatives for one namespace! Actual Ns: " + actualNs + " second alternative: " + alternativeNs);
      }
      actualNsToAlternative.put(actualNs, alternativeNs);
      alternativeNsToActual.put(alternativeNs, actualNs);
    }
docs: Declares an alternative namespace for an actual so that during lookup of elements/attributes both will be considered.
  This can be used if a newer namespaces replaces an older one but XML files with the old one should still be parseable.
  @param alternativeNs
  @param actualNs
  @throws IllegalArgumentException if the alternative is already used or if the actual namespace has an alternative