def era(YY, m, n, nin, nout, r):
    """
    Calculate an ERA model of order `r` based on the impulse-response data `YY`.

    .. note:: This function is not implemented yet.

    Parameters
    ----------
    YY: array
        `nout` x `nin` dimensional impulse-response data
    m: integer
        Number of rows in Hankel matrix
    n: integer
        Number of columns in Hankel matrix
    nin: integer
        Number of input variables
    nout: integer
        Number of output variables
    r: integer
        Order of model

    Returns
    -------
    sys: StateSpace
        A reduced order model sys=ss(Ar,Br,Cr,Dr)

    Examples
    --------
    >>> rsys = era(YY, m, n, nin, nout, r)
    """
    raise NotImplementedError('This function is not implemented yet.')
