public void RegisterRemoveNewExpressionAssignment(PropGraphNodeDescriptor lhs)
        {
            if (lhs != null)
            {
                var types = PropagationGraph.GetTypes(lhs);

                this.RemovedTypes.UnionWith(types);

                PropagationGraph.RemoveTypes(lhs, types);
                PropagationGraph.AddToDeletionWorkList(lhs);
            }
        }