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: position.go
    path of file: ./repos/kubernetes/vendor/gopkg.in/gcfg.v1/token
    
    the code of the file until where you have to start completion: // Copyright 2010 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.

// TODO(gri) consider making this a separate package outside the go directory.

package token

import (
	"fmt"
	"sort"
	"sync"
)

// -----------------------------------------------------------------------------
// Positions

// Position describes an arbitrary source position

    
func (s *FileSet) AddFile(filename string, base, size int) *File {
	s.mutex.Lock()
	defer s.mutex.Unlock()
	if base < s.base || size < 0 {
		panic("illegal base or size")
	}
	// base >= s.base && size >= 0
	f := &File{s, filename, base, size, []int{0}, nil}
	base += size + 1 // +1 because EOF also has a position
	if base < 0 {
		panic("token.Pos offset overflow (> 2G of source code in file set)")
	}
	// add the file to the file set
	s.base = base
	s.files = append(s.files, f)
	s.last = f
	return f
}