const yes = choice(1)
const no = choice(0)

// ctMask is all 1s if on is yes, and all 0s otherwise.
func ctMask(on choice) uint { return -uint(on) }

// ctEq returns 1 if x == y, and 0 otherwise. The execution time of this
// function does not depend on its inputs.
func ctEq(x, y uint) choice {
	// If x != y, then either x - y or y - x will generate a carry.
	_, c1 := bits.Sub(x, y, 0)
	_, c2 := bits.Sub(y, x, 0)