def _transform_side(side: str) -> str:
    """Transform given keyword (eg 'right') to search string (eg 'R_')."""
    if side == "right":
        return "R_"
    if side == "left":
        return "L_"
    raise ValueError(
        f"Invalid argument for `side`. Must be right " f"or left. Got: {side}."
    )