def _cubic_bernstein_extrema(p0, p1, p2, p3):
        """
        Find extremas of a function of real domain defined by evaluating
        a cubic bernstein polynomial of given bernstein coefficients.
        """
        # compute coefficients of derivative
        a = 3.*(p3-p0+3.*(p1-p2))
        b = 6.*(p0+p2-2.*p1)
        c = 3.*(p1-p0)

        if a == 0:
            if b == 0:
                return ()  # constant
            return (-c / b,)  # linear

        # quadratic
        # compute discriminant
        d = b*b - 4.*a*c
        if d < 0:
            return ()

        k = -2. * a
        if d == 0:
            return (b / k,)

        r = math.sqrt(d)
        return ((b + r) / k, (b - r) / k)
Find extremas of a function of real domain defined by evaluating
        a cubic bernstein polynomial of given bernstein coefficients.