code: public T namespace(String prefix, String namespaceUri) {
          getXpathVariableExtractor().getNamespaces().put(prefix, namespaceUri);
          xmlMessageValidationContext.getNamespaces().put(prefix, namespaceUri);
          return self;
      }
docs: Adds explicit namespace declaration for later path validation expressions.
  @param prefix
  @param namespaceUri
  @return