pub fn p_minus_step(
v: [b85; 3],
acc: [b85; 3],
c: bool,
step: b2,
) -> ([b85; 3], bool)Expand description
One limb of p - v = !v + (2^255 - 18) mod 2^255: limb step of
the result from limb step of v and the carry out of the limbs below
(c); the other limbs of acc pass through. One 85-bit carry chain.