Write a method called compress that replaces every pair of elements in the list with a single element equal to the sum of the pair. If the list is of odd size, leave the last element unchanged. For example, if the list stores [1, 7, 3, 9, 4, 6, 5], your method should change it to store [8, 12, 10, 5] (1 + 7, and 3 + 9, then 4 + 6, then 5).