
    Transform a continuous to a discrete state-space system.

    Parameters
    ----------
    system : a tuple describing the system or an instance of `lti`
        The following gives the number of elements in the tuple and
        the interpretation:

            * 1: (instance of `lti`)
            * 2: (num, den)
            * 3: (zeros, poles, gain)
            * 4: (A, B, C, D)

    dt : float
        The discretization time step.
    method : {"gbt", "bilinear", "euler", "backward_diff", "zoh"}, optional
        Which method to use:

            * gbt: generalized bilinear transformation
            * bilinear: Tustin's approximation ("gbt" with alpha=0.5)
            * euler: Euler (or forward differencing) method ("gbt" with alpha=0)
            * backward_diff: Backwards differencing ("gbt" with alpha=1.0)
            * zoh: zero-order hold (default)

    alpha : float within [0, 1], optional
        The generalized bilinear transformation weighting parameter, which
        should only be specified with method="gbt", and is ignored otherwise

    Returns
    -------
    sysd : tuple containing the discrete system
        Based on the input type, the output will be of the form

        * (num, den, dt)   for transfer function input
        * (zeros, poles, gain, dt)   for zeros-poles-gain input
        * (A, B, C, D, dt) for state-space system input

    Notes
    -----
    By default, the routine uses a Zero-Order Hold (zoh) method to perform
    the transformation.  Alternatively, a generalized bilinear transformation
    may be used, which includes the common Tustin's bilinear approximation,
    an Euler's method technique, or a backwards differencing technique.

    The Zero-Order Hold (zoh) method is based on [1]_, the generalized bilinear
    approximation is based on [2]_ and [3]_.

    References
    ----------
    .. [1] http://en.wikipedia.org/wiki/Discretization#Discretization_of_linear_state_space_models

    .. [2] http://techteach.no/publications/discretetime_signals_systems/discrete.pdf

    .. [3] G. Zhang, X. Chen, and T. Chen, Digital redesign via the generalized
        bilinear transformation, Int. J. Control, vol. 82, no. 4, pp. 741-754,
        2009.
        (http://www.ece.ualberta.ca/~gfzhang/research/ZCC07_preprint.pdf)

    