actContents()
    // and deleteContents(), we just reuse it here, on the theory of "why
    // not test some more stuff if it's easy to do".
    //
    // Just to be pedantic, we'll test not only that the tree we're
    // modifying is the same in expected vs. actual, but also that all the
    // nodes originally in it were the same.  Typically some nodes will
    // become detached when the algorithm is run, but they still exist and
    // references can still be kept to them, so they should also remain th