Sorry, I seemed to misunderstand the difference of file and document. I will be more careful next time.
I’m talking about this section of API document
https://apidocs.dynalist.io/#make-changes-to-documents-and-folders
specifically, the following 2 parts:
I was trying to move a node from its current parent_node to another parent_node.
I followed the API document’s descriptions at first but not succeeded. After some trial-and-error, I found that the following json syntax works:
That means, there are at least 2 errors or inaccurate descriptions in API document:
1. to move a node, one has to explicitly specify the root_node’s “file_id”, besides “token” and “changes”
2. to move a node, one should use “node_id” instead of “file_id” inside “changes” sub-section
I hope I’m not mistaken. 
Sorry, I seemed to misunderstand the difference of file and document. I will be more careful next time.