def in_adjacent(filter=nil, &block)
      r = self.collect {|v| v.in_adjacent(filter, &block) }
      VertexSet.new(r).flatten.uniq
    end
def in_adjacent(filter=nil, &block)
      r = self.collect {|v| v.in_adjacent(filter, &block) }
      VertexSet.new(r).flatten.uniq
    end
Returns all back-adjacent vertices reachable from this set