def single_pairwise(arr, arg, taken):
    """Get a single index pair (i, j), where
    arr[i] + arr[j] == arg. 

    Arguments:
        arr: an array of numbers
        arg: the target sum to look for
        taken: an array as long as arr designating
            if a particular index can be in a new
            index pair.
    """
    i = -1
    j = -1

    # Compare the sum of every pair of values in
    # arr to arg, and return the first set of
    # indices where the pair of values add to arg.
    for i in range(len(arr)):
        for j in range(i + 1, len(arr)):
            if arr[i] + arr[j] == arg and taken[i] == taken[j] == False:
                return (i, j)

    # If no pair of values sum to arg, then we
    # indicate that with special value -1.
    return (-1, -1)