pub fn nb_const(sub: bool, low: bool) -> b85
A limb (the low one or a higher one) of the constant added to x (b, or !b for subtraction) to form nb: !b + (2^255 - 18) = p - b mod 2^255, or b + 0.
low
x
b
!b
nb
!b + (2^255 - 18) = p - b
2^255
b + 0