@Override
  public Object getVariableValue(String namespaceURI, String prefix,
                                 String localName) throws UnresolvableException {
    if (namespaceURI == null) {
      namespaceURI = "";
    }
    if (prefix == null) {
      prefix = "";
    }
    try {
      if (namespaceURI.isEmpty()) {
        namespaceURI = getNamespace(prefix).getURI();
      }
      return getVariable(localName, Namespace.getNamespace(namespaceURI));
    }
    catch (IllegalArgumentException e) {
      throw new UnresolvableException("Unable to resolve variable " +
                                      localName + " in namespace '" + namespaceURI +
                                      "' to a vaulue.");
    }
  }
@Override
  public Object getVariableValue(String namespaceURI, String prefix,
                                 String localName)