def _get_live_channels(Xs, channel_indices=None):
    """
    Determine which channels are not None.

    Parameters
    ----------
    Xs : list
        List of feature matrices and None placeholders.
    channel_indices : int, list/array of ints, or None, default=None
        - If int or list/array of int: Indices of the matrices in Xs to query.
        - If None: Query all input matrices in Xs.

    Returns
    -------
    list
        List of indices into Xs where the value is not None.
    """
    if channel_indices is None:
        channel_indices = range(len(Xs))
    if type(channel_indices) == int:
        if Xs[channel_indices] is None:
            live_channels = []
        else:
            live_channels = [channel_indices]
    else:
        live_channels = [i for i in channel_indices if Xs[i] is not None]
    return live_channels