Trait std::ops::Neg

use std::ops::Neg;

#[derive(Debug, PartialEq)]
enum Sign {
    Negative,
    Zero,
    Positive,
}

impl Neg for Sign {
    type Output = Self;

    fn neg(self) -> Self::Output {
        match self {
            Sign::Negative => Sign::Positive,
            Sign::Zero => Sign::Zero,
            Sign::Positive => Sign::Negative,
        }
    }
}

// A negative positive is a negative.
assert_eq!(-Sign::Positive, Sign::Negative);
// A double negative is a positive.
assert_eq!(-Sign::Negative, Sign::Positive);
// Zero is its own negation.
assert_eq!(-Sign::Zero, Sign::Zero);