private IEnumerable <FormNodeSelector> SelectRemovableNodesByForm(IReadOnlyStatGraph statGraph) =>
 from pair in statGraph.FormNodeCollections
     where _nodeRemovalDeterminer.CanBeRemoved(pair.Value)
 select pair.Key;
 private bool CanStatGraphBeRemoved(IReadOnlyStatGraph statGraph) =>
 statGraph.Nodes.IsEmpty() && statGraph.FormNodeCollections.IsEmpty() &&
 _nodeRemovalDeterminer.CanBeRemoved(statGraph.Paths);
 private IEnumerable <NodeSelector> SelectRemovableNodesByNodeType(IReadOnlyStatGraph statGraph) =>
 from pair in statGraph.Nodes
 orderby pair.Key.NodeType
     where _nodeRemovalDeterminer.CanBeRemoved(pair.Value)
 select pair.Key;