montgomerySq

        fn montgomerySq(self: Self, x: Fe) Fe {
            var d = self.zero;
            assert(x.limbs_count() == self.limbs_count());
            const overflow = self.montgomeryLoop(&d, x, x);
            const underflow = 1 -% @intFromBool(ct.limbsCmpGeq(d.v, self.v));
            const need_sub = ct.eql(overflow, underflow);
            _ = d.v.conditionalSubWithOverflow(need_sub, self.v);
            d.montgomery = true;
            return d;
        }