code: public ImmutableNamespaces withPrefix(String prefix, String namespaceURI) {
  		Objects.requireNonNull(prefix, "Prefix cannot be null");
  		Objects.requireNonNull(namespaceURI, "Namespace URI cannot be null");
  		ImmutableNamespaces result=new ImmutableNamespaces(this.map);
  		result.map.put(prefix, namespaceURI);
  		return result;
  	}
docs: Create a copy of the namespaces which includes a mapping between a given
  prefix and namespace URI.
  
  @param prefix
  the prefix to be added
  @param namespaceURI
  the namespace URI to be mapped to the prefix
  @return a copy of the instance that includes the mapping between the
  specified namespace URI and prefix.
  @throws NullPointerException if the prefix or namespace URI are null.