uint32(constantTimeSelectLe(x, q/2, x, q-x)) } // fieldReduceOnce reduces a value a < 2q. func fieldReduceOnce(a uint32) fieldElement { x, b := bits.Sub64(uint64(a), uint64(q), 0) return fieldElement(x + b*q) } // fieldAdd returns a + b mod q. func fieldAdd(a, b fieldElement) fieldElement { x := uint32(a + b) return fieldReduceOnce(x) } // fieldSub returns a - b mod q. func fieldSub(a, b fieldElement) fieldElement { x := uint32(a - b + q) return fieldReduceOnce(x) } // fieldMontgomeryMul returns a * b * R⁻¹ mod q....