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: repo.go
    path of file: ./repos/sliver/vendor/github.com/Binject/shellcode
    
    the code of the file until where you have to start completion: package shellcode

import (
	"errors"
	"io/ioutil"
	"log"
	"os"
	"path/filepath"

	"github.com/Binject/shellcode/api"
)

// Repo - Shellcode R
    
func (r *Repo) Lookup(os api.Os, arch api.Arch, pattern string) ([]byte, error) {
	if r.Directory != "" {
		// check specific directory first
		dir := filepath.Join(r.Directory, string(os), string(arch))
		if DirExists(dir) {
			files, err := WalkMatch(dir, pattern)
			if err != nil {
				return nil, err
			}
			if len(files) > 0 {
				return ioutil.ReadFile(files[0])
			}
		}
		// todo: fallback from intel32 or 64 to 32y64
	}
	//todo: lookup in the compiled-in modules
	return nil, errors.New("No Matching Shellcode Found")
}