反馈节点集FNS ？
最小反馈节点集问题是一个NP完全问题，很难准确求解。
As the minimum feedback vertex set problem is NP-complete, it is hard to be solved exactly.
另一个原因是，当用户创建连接时，连接EditPart是不存在的，因此节点必须能够自己显示反馈。
Another reason is that when the user is creating a connection, the connection EditPart doesn't exist, so the node must be able to show feedback on its own.
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