final ObjectWriter ow = new ObjectWriter(db);
A_A.setId(ow.writeTree(A_A));
A_B.setId(ow.writeTree(A_B));
A.setId(ow.writeTree(A));
root.setId(ow.writeTree(root));
tree_root = rw.parseTree(root.getId());
tree_A = rw.parseTree(A.getId());
tree_AB = rw.parseTree(A_A.getId());
assertSame(tree_AB, rw.parseTree(A_B.getId()));