def bench2(x):
    """A benchmark function for test purposes.

        f(x) = x ** 2           if x < 0
               (x-5) ** 2 - 5   otherwise.

    It has a global minima with f(x*) = -5 at x* = 5.
    """
    if x[0] < 0:
        return x[0] ** 2
    else:
        return (x[0] - 5) ** 2 - 5
