def mae_expr(gray_only: bool = True) -> str:
    """Mean Absolute Error string to be integrated in std.Expr.

    Args:
        gray_only (bool, optional):
            If both actual observation and prediction are one plane each.
            Defaults to True.

    Returns:
        str: Expression.
    """
    return 'x y - abs' if gray_only else 'x a - abs y b - abs max z c - abs max'