/**
     * Merges two arrays as needed by the Merge Sort algorithm.<br />
     * <br />
     * <b>Preconditions</b>:
     * <ul>
     *     <li>0 &lt;= middle &lt; a.length
     *     <li>Array <code>arr</code> has two parts that have already been sorted:
     *         (1) From the beginning to index <code>middle</code>
     *         (2) From index <code>middle + 1</code> to the end of the array.
     * </ul>
     * <br />
     * <b>Postconditions</b>:
     * <ul>
     *     <li>Array <code>arr</code> is completely sorted
     * </ul>
     * @param arr Voodoo.  Wait, what?
     * @param middle Voodoo.  Wait, what?
     */