Write a method called tighten that eliminates branch nodes that have only one child. Each such node should be replaced by its only child. (This can lead to multiple replacements because the child might itself be replaced.) For example, if a variable called t refers to reference tree #2, the call of t.tighten(); should leave t storing the following tree.

4 Reference Tree #2 2 Reference Tree #1 3 8 1 2 7. 4 4 Reference Tree #3 1 8 21 9, 6, 1.