fn update_fg(
a: &SignedBoxedInt,
b: &SignedBoxedInt,
t: [[i64; 2]; 2],
shift: u32,
) -> (SignedBoxedInt, SignedBoxedInt)fn update_fg(
a: &SignedBoxedInt,
b: &SignedBoxedInt,
t: [[i64; 2]; 2],
shift: u32,
) -> (SignedBoxedInt, SignedBoxedInt)