able of the form ``extra_condition(alpha, phi_value)``
        returning a boolean. The line search accepts the value
        of ``alpha`` only if this callable returns ``True``.
        If the callable returns ``False`` for the step length,
        the algorithm will continue with new iterates.
        The callable is only called for iterates satisfying
        the strong Wolfe conditions.
    maxiter : int, optional
        Maximum number of iterations to perform.

    Returns
    -------
    alpha_star : float or None
        Best alpha, or None if the line search algorithm did not converge.
    phi_star : float
        phi at alpha_star.
    phi0 : float
        phi at 0.
    derphi_star : float or None
        derphi at alpha_star, or None if the line search algorithm
        did not converge.

    Notes
    -----
    Uses the line search algorithm to enforce strong Wolfe
    conditions. See Wright and Nocedal, 'Numerical Optimization',
    1999, pp. 59-61.

    Nr6