def getABMN(scheme, idx):
    """ Get coordinates of four-point cfg with id `idx` from DataContainerERT
    `scheme`."""
    coords = {}
    for elec in "abmn":
        elec_id = int(scheme(elec)[idx])
        elec_pos = scheme.sensorPosition(elec_id)
        coords[elec] = elec_pos.x(), elec_pos.y()
    return coords