y := rndW()
		z := make(nat, n)
		arg := argVW{z, x, y, 0}
		testFunVWext(t, "addVW, random inputs", addVW, addVW_g, arg)
		testFunVWext(t, "subVW, random inputs", subVW, subVW_g, arg)

		// vector of random numbers, but make 'x' and 'z' share storage
		arg = argVW{x, x, y, 0}
		testFunVWext(t, "addVW, random inputs, sharing storage", addVW, addVW_g, arg)
		testFunVWext(t, "subVW, random inputs, sharing storage", subVW, subVW_g, arg)

	STP	(R10, R11), -16(R3);	\
	SUB	$4, counter;

// do one iteration of copy in addVW/subVW
#define vwOneIterCopy(counter, exit)			\
	CBZ	counter, exit;				\
	LDP.P	32(R1), (R4, R5);			\
	LDP	-16(R1), (R6, R7);			\
	STP.P	(R4, R5), 32(R3);			\
	STP	(R6, R7), -16(R3);			\
	SUB	$4, counter;

// func addVW(z, x []Word, y Word) (c Word)
// The 'large' branch handles large 'z'. It checks the carry flag on every iteration

	MOVD  8(R8), R11
	MOVD  8(R9), R16
	SUBE  R16, R11, R20
	MOVD  R20, 8(R10)

final:
	ADDZE R4
	XOR   $1, R4

done:
	MOVD  R4, c+72(FP)
	RET

// func addVW(z, x []Word, y Word) (c Word)
TEXT ·addVW(SB), NOSPLIT, $0
	MOVD z+0(FP), R10	// R10 = z[]
	MOVD x+24(FP), R8	// R8 = x[]
	MOVD y+48(FP), R4	// R4 = y = c
	MOVD z_len+8(FP), R11	// R11 = z_len

			"flag.kind",
			"flag.ro",
		},
		"regexp": {
			"(*bitState).push",
		},
		"math/big": {
			"bigEndianWord",
			// The following functions require the math_big_pure_go build tag.
			"addVW",
			"subVW",
		},
		"math/rand": {
			"(*rngSource).Int63",
			"(*rngSource).Uint64",
		},
		"net": {
			"(*UDPConn).ReadFromUDP",
		},
		"sync": {

		// n == 0 because m >= n; result is 0
		return z[:0]
	case n == 0:
		// result is x
		return z.set(x)
	}
	// m > 0

	z = z.make(m + 1)
	c := addVV(z[0:n], x, y)
	if m > n {
		c = addVW(z[n:m], x[n:], c)
	}
	z[m] = c

	return z.norm()
}

func (z nat) sub(x, y nat) nat {
	m := len(x)
	n := len(y)

	switch {
	case m < n:
		panic("underflow")
	case m == 0:

	MOVD R0, R4
	SUBE R4, R4            // save CF

	ADD $8, R10 // i++
	SUB $1, R3  // n--
	BGT L1      // if n > 0 goto L1

E1:
	NEG  R4, R4
	MOVD R4, c+72(FP) // return c
	RET

TEXT ·addVW(SB), NOSPLIT, $0
	MOVD z_len+8(FP), R5 // length of z
	MOVD x+24(FP), R6
	MOVD y+48(FP), R7    // c = y
	MOVD z+0(FP), R8

	CMPBEQ R5, $0, returnC // if len(z) == 0, we can have an early return

		// and it's Below if we truncate (Exact results require no rounding).
		// For a negative result (z.neg) it is exactly the opposite.
		z.acc = makeAcc(inc != z.neg)

		if inc {
			// add 1 to mantissa
			if addVW(z.mant, z.mant, lsb) != 0 {
				// mantissa overflow => adjust exponent
				if z.exp >= MaxExp {
					// exponent overflow
					z.form = inf
					return
				}
				z.exp++