def from_graphxml(source)
      listener = MutableGraphParser.new(self)
      REXML::Document.parse_stream(source, listener)
      self
    end
def from_graphxml(source)
      listener = MutableGraphParser.new(self)
      REXML::Document.parse_stream(source, listener)
      self
    end
class MutableGraphParser
 Initializes an RGL graph from a subset of the GraphML format given in
 +source+ (see http://www.graphdrawing.org/graphml).