fn test(instructions: &[Instruction], registers: [u32; 4]) -> u32 {
    if let Some(&instruction) = instructions.get(0) {
        match instruction {
            Instruction::A(a) => registers[a as usize],
            Instruction::B(b) => registers[b as usize],
        }
    } else {
        0
    }
}

#[derive(Copy, Clone)]
enum Instruction {
    A(Register),
    B(Register),
}

#[derive(Copy, Clone)]
enum Register {
    A,
    B,
    C,
    D,
}