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: message_set.go
    path of file: ./repos/fabio/vendor/github.com/gogo/protobuf/proto
    
    the code of the file until where you have to start completion: // Go support for Protocol Buffers - Google's data interchange format
//
// Copyright 2010 The Go Authors.  All rights reserved.
// https://github.com/golang/protobuf
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
/
    
func unmarshalMessageSet(buf []byte, exts interface{}) error {
	var m map[int32]Extension
	switch exts := exts.(type) {
	case *XXX_InternalExtensions:
		m = exts.extensionsWrite()
	case map[int32]Extension:
		m = exts
	default:
		return errors.New("proto: not an extension map")
	}

	ms := new(messageSet)
	if err := Unmarshal(buf, ms); err != nil {
		return err
	}
	for _, item := range ms.Item {
		id := *item.TypeId
		msg := item.Message

		// Restore wire type and field number varint, plus length varint.
		// Be careful to preserve duplicate items.
		b := EncodeVarint(uint64(id)<<3 | WireBytes)
		if ext, ok := m[id]; ok {
			// Existing data; rip off the tag and length varint
			// so we join the new data correctly.
			// We can assume that ext.enc is set because we are unmarshaling.
			o := ext.enc[len(b):]   // skip wire type and field number
			_, n := DecodeVarint(o) // calculate length of length varint
			o = o[n:]               // skip length varint
			msg = append(o, msg...) // join old data and new data
		}
		b = append(b, EncodeVarint(uint64(len(msg)))...)
		b = append(b, msg...)

		m[id] = Extension{enc: b}
	}
	return nil
}