pub fn limb_sub(a: &[u64; 4], b: &[u64; 4]) -> ([u64; 4], u64)Expand description
The software limb-subtract — mirror of mag_sub’s borrow chain, but total (defined for
every a, b): the low LIMBS difference limbs and the borrow-out (1 iff a < b, i.e. the
subtraction underflowed). When borrow == 0 the difference equals mag_sub(a, b).