def in_edges(filter=nil, &block)
      r = self.collect {|v| v.in_edges(filter, &block) }
      EdgeSet.new(r).flatten.uniq
    end
def in_edges(filter=nil, &block)
      r = self.collect {|v| v.in_edges(filter, &block) }
      EdgeSet.new(r).flatten.uniq
    end
Walking section 
 Returns incoming edges of all vertices of this set