th call signature
        `y = matvec(x)`, where `x` and `y` are the input and output vectors,
        respectively.
    :type matvec: function
    :param k:
        Rank of SVD.
    :type k: int

    :return:
        Left singular vectors.
    :rtype: :class:`numpy.ndarray`
    :return:
        Right singular vectors.
    :rtype: :class:`numpy.ndarray`
    :return:
        Singular values.
    :rtype: :class:`numpy.ndarray`
    )