"**Example:** symmetrical_sum([10,8,7,5,9,8,15]) == ([8, 7, 5, 9, 8], 37). Here a symmetrical portion of the list is formed by the elements at the second and second-last index, which share a value of ...
You are given a sorted array consisting of only integers where every element appears exactly twice, except for one element which appears exactly once. Return the single element that appears only once.