code: func (g *Graph) DiscardLowFrequencyNodePtrs(nodeCutoff int64) NodePtrSet {
  	cutNodes := getNodesAboveCumCutoff(g.Nodes, nodeCutoff)
  	kept := make(NodePtrSet, len(cutNodes))
  	for _, n := range cutNodes {
  		kept[n] = true
  	}
  	return kept
  }
docs: // DiscardLowFrequencyNodePtrs returns a NodePtrSet of nodes at or over a
  // specific cum value cutoff.