Why is the below [3,7] instead of [5,5]?
Doesn't [5,5] occur first if we start from the left?

sum_pairs([10, 5, 2, 3, 7, 5],         10)
#              ^-----------^   5 + 5 = 10, indices: 1, 5
#                    ^--^      3 + 7 = 10, indices: 3, 4 *
#  * entire pair is earlier, and therefore is the correct answer
== [3, 7]

Oh my..how does one even think of that