func isLetter(ch rune) bool  { return 'a' <= lower(ch) && lower(ch) <= 'z' || ch == '_' }
func isDecimal(ch rune) bool { return '0' <= ch && ch <= '9' }
func isHex(ch rune) bool     { return '0' <= ch && ch <= '9' || 'a' <= lower(ch) && lower(ch) <= 'f' }

// digits accepts the sequence { digit | '_' }.
// If base <= 10, digits accepts any decimal digit but records

		ch = s.next()
	}
	return ch
}

func lower(ch rune) rune     { return ('a' - 'A') | ch } // returns lower-case ch iff ch is ASCII letter
func isDecimal(ch rune) bool { return '0' <= ch && ch <= '9' }
func isHex(ch rune) bool     { return '0' <= ch && ch <= '9' || 'a' <= lower(ch) && lower(ch) <= 'f' }

// digits accepts the sequence { digit | '_' } starting with ch0.
// If base <= 10, digits accepts any decimal digit but records

	}
	for i := 0; i < len(s); {
		switch s[i] {
		case ';':
			return reencode(s)
		case '%':
			if i+2 >= len(s) || !ishex(s[i+1]) || !ishex(s[i+2]) {
				return reencode(s)
			}
			i += 3
		default:
			i++
		}
	}
	return s
}

func ishex(c byte) bool {
	switch {
	case '0' <= c && c <= '9':
		return true
	case 'a' <= c && c <= 'f':
		return true

	return 16 // larger than any legal digit val
}

func lower(ch rune) rune     { return ('a' - 'A') | ch } // returns lower-case ch iff ch is ASCII letter
func isDecimal(ch rune) bool { return '0' <= ch && ch <= '9' }
func isHex(ch rune) bool     { return '0' <= ch && ch <= '9' || 'a' <= lower(ch) && lower(ch) <= 'f' }

// digits accepts the sequence { digit | '_' }.
// If base <= 10, digits accepts any decimal digit but records

	// Count %, check that they're well-formed.
	n := 0
	hasPlus := false
	for i := 0; i < len(s); {
		switch s[i] {
		case '%':
			n++
			if i+2 >= len(s) || !ishex(s[i+1]) || !ishex(s[i+2]) {
				s = s[i:]
				if len(s) > 3 {
					s = s[:3]
				}
				return "", EscapeError(s)
			}
			// Per https://tools.ietf.org/html/rfc3986#page-21

	PrefixVEX2Bytes Prefix = 0xC5 // Short form of vex prefix
	PrefixVEX3Bytes Prefix = 0xC4 // Long form of vex prefix
)

// IsREX reports whether p is a REX prefix byte.
func (p Prefix) IsREX() bool {
	return p&0xF0 == PrefixREX
}

func (p Prefix) IsVEX() bool {
	return p&0xFF == PrefixVEX2Bytes || p&0xFF == PrefixVEX3Bytes
}

func (p Prefix) String() string {

		for i, p := range inst.Prefix {
			switch p &^ PrefixIgnored {
			case PrefixData16, PrefixData32, PrefixCS, PrefixDS, PrefixES, PrefixSS:
				inst.Prefix[i] |= PrefixImplicit
			}
			if p.IsREX() {
				inst.Prefix[i] |= PrefixImplicit
			}
			if p.IsVEX() {
				if p == PrefixVEX3Bytes {
					inst.Prefix[i+2] |= PrefixImplicit
				}
				inst.Prefix[i] |= PrefixImplicit
				inst.Prefix[i+1] |= PrefixImplicit
			}
		}
	}

			implicitData = true
		}
	}
	for _, p := range inst.Prefix {
		if p == 0 || p.IsVEX() {
			break
		}
		if p&PrefixImplicit != 0 {
			continue
		}
		switch p &^ (PrefixIgnored | PrefixInvalid) {
		default:
			if p.IsREX() {
				if p&0xFF == PrefixREX {
					prefix += "rex "
				} else {
					prefix += "rex." + p.String()[4:] + " "
				}
				break

			for j := 0; j < n; j++ {
				prefix := Prefix(decoder[pc+2*j])
				if prefix.IsREX() {
					rexUsed |= prefix
					if rex&prefix == prefix {
						pc = int(decoder[pc+2*j+1])
						continue Decode
					}
					continue
				}
				ok := false
				if prefix == 0 {
					ok = true
				} else if prefix.IsREX() {
					rexUsed |= prefix
					if rex&prefix == prefix {
						ok = true
					}