def each_edge(filter=nil, &block)
      if filter.nil?
        @edges.each &block
      else
        edges(filter).each &block
      end
    end
def each_edge(filter=nil, &block)
      if filter.nil?
        @edges.each &block
      else
        edges(filter).each &block
      end
    end
Calls block on each graph edge for with the 'filter and block' predicate
 evaluates to true.