[Thorrez]

Here's the new code:

    beta8.Mod(beta8, curve.P)
    x3.Sub(x3, beta8)
    if x3.Sign() == -1 {
        x3.Add(x3, curve.P)
    }
    x3.Mod(x3, curve.P)

I don't understand it why it's all necessary. This is shorter and seems to do the exact same thing:

    beta8.Mod(beta8, curve.P)
    x3.Sub(x3, beta8)
    x3.Mod(x3, curve.P)

Also, beta8 is never used after this code. So this should do the same thing as well:

    x3.Sub(x3, beta8)
    x3.Mod(x3, curve.P)

[lifthrasiir]

I think you are right. Go `big.Mod` should be Euclidean (i.e. `x % y` follows `y`'s sign) so the code is redundant. It doesn't seem to be required to run in constant time (if so we won't have `if` at all), probably the committer wanted a minimal change?