subtract method
Subtract sets v = a - b, and returns v.
Implementation
void subtract(Element a, Element b) {
// We first add 2 * p, to guarantee the subtraction won't underflow, and
// then subtract b (which can be up to 2^255 + 2^13 * 19).
l0 = (a.l0 + _fFFFFFFFFFFDA) - b.l0;
l1 = (a.l1 + _fFFFFFFFFFFFE) - b.l1;
l2 = (a.l2 + _fFFFFFFFFFFFE) - b.l2;
l3 = (a.l3 + _fFFFFFFFFFFFE) - b.l3;
l4 = (a.l4 + _fFFFFFFFFFFFE) - b.l4;
carryPropagateGeneric();
}