def rdf_node_from(source)
      case source
      when NilClass
        nil
      when Node
        Redland.librdf_new_node_from_node(source.rdf_node)
      else
        Redland.librdf_new_node_from_node(Node.new(source).rdf_node)
      end
    end
def rdf_node_from(source)
      case source
      when NilClass
        nil
      when Node
        Redland.librdf_new_node_from_node(source.rdf_node)
      else
        Redland.librdf_new_node_from_node(Node.new(source).rdf_node)
      end
    end
Create a Node from the source
 and get its rdf_node, or return nil
 @api private