def _copy_channel(inst, ch_name, new_ch_name):
    """Add a copy of a channel specified by ch_name.

    Input data can be in the form of Raw, Epochs or Evoked.

    The instance object is modified inplace.

    Parameters
    ----------
    inst : instance of Raw | Epochs | Evoked
        Data containing the EEG channels
    ch_name : str
        Name of the channel to copy.
    new_ch_name : str
        Name given to the copy of the channel.

    Returns
    -------
    inst : instance of Raw | Epochs | Evoked
        The data with a copy of a given channel.
    """
    new_inst = inst.copy().pick_channels([ch_name])
    new_inst.rename_channels({ch_name: new_ch_name})
    inst.add_channels([new_inst], force_update_info=True)
    return inst