select method
Select sets v to a if cond == 1, and to b if cond == 0.
Implementation
void select(Element a, Element b, int cond) {
final BigInt m = mask64Bits(cond);
l0 = (m & a.l0) | (~m & b.l0);
l1 = (m & a.l1) | (~m & b.l1);
l2 = (m & a.l2) | (~m & b.l2);
l3 = (m & a.l3) | (~m & b.l3);
l4 = (m & a.l4) | (~m & b.l4);
}