def sum_series(n, n0=0, n1=1):
    """
    Compute the nth value of a summation series.

    :param n0=0: value of zeroth element in the series
    :param n1=1: value of first element in the series

    This function should generalize the fibonacci() and the lucas(),
    so that this function works for any first two numbers for a sum series.
    Once generalized that way, sum_series(n, 0, 1) should be equivalent to fibonacci(n).
    And sum_series(n, 2, 1) should be equivalent to lucas(n).

    sum_series(n, 3, 2) should generate antoehr series with no specific name

    The defaults are set to 0, 1, so if you don't pass in any values, you'll
    get the fibonacci sercies
    """

    if n < 0:
        return None
    if n == 0:
        return n0
    elif n == 1:
        return n1
    else:
        return sum_series(n - 1, n0, n1) + sum_series(n - 2, n0, n1)