prospective copied node 准复制节点
If we descend to the left, we point the previously copied node's left child to the currently copied node. The previous node's right child continues to point to nodes in the older version.
如果转向左边，需要将上一个拷贝的节点指向当前拷贝节点，而前一个节点的右侧节点则继续指向原来旧版本的节点。
The dragged port type node is copied to the target tree along with all the operations it contains.
将被拖放的端口类型结点复制到目标树中，连同其中包含的所有操作。
The destination node may be in another document, in which case the subtree is copied between documents.
目标节点可以在另一个文档中，在这种情况下，在两个文档之间复制子树。
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