// Operate on numbers of different precision.
	var x, y, z big.Float
	x.SetInt64(1000)          // x is automatically set to 64bit precision
	y.SetFloat64(2.718281828) // y is automatically set to 53bit precision
	z.SetPrec(32)
	z.Add(&x, &y)
	fmt.Printf("x = %.10g (%s, prec = %d, acc = %s)\n", &x, x.Text('p', 0), x.Prec(), x.Acc())
	fmt.Printf("y = %.10g (%s, prec = %d, acc = %s)\n", &y, y.Text('p', 0), y.Prec(), y.Acc())

	for _, prec := range precList {
		two := new(Float).SetPrec(prec).SetInt64(2)
		one := new(Float).SetPrec(prec).SetInt64(1)
		three := new(Float).SetPrec(prec).SetInt64(3)
		msix := new(Float).SetPrec(prec).SetInt64(-6)
		psix := new(Float).SetPrec(prec).SetInt64(+6)

		p := new(Float).SetPrec(prec)
		z1 := new(Float).SetPrec(prec)
		z2 := new(Float).SetPrec(prec)

		// z1 = 2 + 1.0/3*-6
		p.Quo(one, three)

					x := sign + test

					var tx Float
					_, _, err := tx.SetPrec(prec).SetMode(mode).Parse(x, 0)
					if err != nil {
						t.Errorf("parsing of %s (%dbits, %v) failed (invalid test case): %v", x, prec, mode, err)
						continue
					}

					// If tx was set to prec == 0, tx.Parse(x, 0) assumes precision 64. Correct it.
					if prec == 0 {
						tx.SetPrec(0)
					}

					if err := enc.Encode(&tx); err != nil {

		err = fmt.Errorf("exponent overflow")
		return
	}

	if exp5 == 0 {
		// no decimal exponent contribution
		z.round(0)
		return
	}
	// exp5 != 0

	// apply 5**exp5
	p := new(Float).SetPrec(z.Prec() + 64) // use more bits for p -- TODO(gri) what is the right number?
	if exp5 < 0 {
		z.Quo(z, p.pow5(uint64(-exp5)))
	} else {
		z.Mul(z, p.pow5(uint64(exp5)))
	}

	return
}

			return errors.New("Float.GobDecode: buffer too small for finite form float")
		}
		z.exp = int32(byteorder.BeUint32(buf[6:]))
		z.mant = z.mant.setBytes(buf[10:])
	}

	if oldPrec != 0 {
		z.mode = oldMode
		z.SetPrec(uint(oldPrec))
	}

	if msg := z.validate0(); msg != "" {
		return errors.New("Float.GobDecode: " + msg)
	}

	return nil
}

// MarshalText implements the [encoding.TextMarshaler] interface.

const (
	// Maximum size in bits for big.Ints before signaling
	// overflow and also mantissa precision for big.Floats.
	ConstPrec = 512
)

func BigFloat(v constant.Value) *big.Float {
	f := new(big.Float)
	f.SetPrec(ConstPrec)
	switch u := constant.Val(v).(type) {
	case int64:
		f.SetInt64(u)
	case *big.Int:
		f.SetInt(u)
	case *big.Float:
		f.Set(u)
	case *big.Rat:
		f.SetRat(u)
	default:

	if mode == ToNearestAway {
		panic("not yet implemented")
	}
	if mode == ToNearestEven && rbit == 1 && (sbit == 1 || sbit == 0 && bit0 != 0) || mode == AwayFromZero {
		// round away from zero
		f.SetMode(ToZero).SetPrec(prec)
		f.Add(f, Bits{int(r) + 1}.Float())
	}
	return f
}

// Float returns the *Float z of the smallest possible precision such that
// z = sum(2**bits[i]), with i = range bits. If multiple bits[i] are equal,

//go:generate stringer -type=Accuracy

// SetPrec sets z's precision to prec and returns the (possibly) rounded
// value of z. Rounding occurs according to z's rounding mode if the mantissa
// cannot be represented in prec bits without loss of precision.
// SetPrec(0) maps all finite values to ±0; infinite values remain unchanged.
// If prec > [MaxPrec], it is set to [MaxPrec].
func (z *Float) SetPrec(prec uint) *Float {

	}

	// round mantissa to n bits
	var n uint
	if prec < 0 {
		n = 1 + (x.MinPrec()-1+3)/4*4 // round MinPrec up to 1 mod 4
	} else {
		n = 1 + 4*uint(prec)
	}
	// n%4 == 1
	x = new(Float).SetPrec(n).SetMode(x.mode).Set(x)

	// adjust mantissa to use exactly n bits
	m := x.mant
	switch w := uint(len(x.mant)) * _W; {
	case w < n:
		m = nat(nil).shl(m, n-w)
	case w > n:
		m = nat(nil).shr(m, w-n)

func (r *Decoder) bigInt() *big.Int {
	v := new(big.Int).SetBytes([]byte(r.String()))
	if r.Bool() {
		v.Neg(v)
	}
	return v
}

func (r *Decoder) bigFloat() *big.Float {
	v := new(big.Float).SetPrec(512)
	assert(v.UnmarshalText([]byte(r.String())) == nil)
	return v
}

// @@@ Helpers

// TODO(mdempsky): These should probably be removed. I think they're a