Unmerge a tree of merged base object records as an intact sub-structure. A sub-tree having unmerged base object records as root will come out from the original merge tree structure. (For example, merge a1 and a2 into a, then merge b1 and b2 into b, and then finally merge a and b into c. If you then perform a tree unmerge on a, and then unmerge a from a1, a2 is a sub tree and will come out from the original tree c. As a result, a is the root of the tree after the unmerge.)