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#
#    groupSum
#
#    Given an array of ints, is it possible to choose a group of some of the ints, such that the
#    group sums to the given target? This is a classic backtracking recursion problem. Once you
#    understand the recursive backtracking strategy in this problem, you can use the same pattern
#    for many problems to search a space of choices. Rather than looking at the whole array, our
#    convention is to consider the part of the array starting at index start and continuing to the
#    end of the array. The caller can specify the whole array simply by passing start as 0. No loops
#    are needed -- the recursive calls progress down the array.
#
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#
#   groupSum(0, {2, 4, 8}, 10) → true
#   groupSum(0, {2, 4, 8}, 14) → true
#   groupSum(0, {2, 4, 8}, 9) → false
#
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