| dm | - the DMPlex object
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| parentSection | - a section describing the tree: a point has a parent if it has 1 dof in the section; the section
offset indexes the parent and childID list; the reference count of parentSection is incremented
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| parents | - a list of the point parents; copied, can be destroyed
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| childIDs | - identifies the relationship of the child point to the parent point; if there is a reference tree, then
the child corresponds to the point in the reference tree with index childIDs; copied, can be destroyed
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