You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: decode.go
    path of file: ./repos/gotosocial/vendor/golang.org/x/arch/x86/x86asm
    
    the code of the file until where you have to start completion: // Copyright 2014 The Go Authors.  All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Table-driven decoding of x86 instructions.

package x86asm

import (
	"encoding/binary"
	"errors"
	"fmt"
	"
    
func instPrefix(b byte, mode int) (Inst, error) {
	// When tracing it is useful to see what called instPrefix to report an error.
	if trace {
		_, file, line, _ := runtime.Caller(1)
		fmt.Printf("%s:%d\n", file, line)
	}
	p := Prefix(b)
	switch p {
	case PrefixDataSize:
		if mode == 16 {
			p = PrefixData32
		} else {
			p = PrefixData16
		}
	case PrefixAddrSize:
		if mode == 32 {
			p = PrefixAddr16
		} else {
			p = PrefixAddr32
		}
	}
	// Note: using composite literal with Prefix key confuses 'bundle' tool.
	inst := Inst{Len: 1}
	inst.Prefix = Prefixes{p}
	return inst, nil
}