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. with java code