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: bmp-string.go
    path of file: ./repos/plumber/vendor/golang.org/x/crypto/pkcs12
    
    the code of the file until where you have to start completion: // Copyright 2015 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.

package pkcs12

import (
	"errors"
	"unicode/utf16"
)

// bmpString returns s encoded in UCS-2 with a zero terminator
    
func bmpString(s string) ([]byte, error) {
	// References:
	// https://tools.ietf.org/html/rfc7292#appendix-B.1
	// https://en.wikipedia.org/wiki/Plane_(Unicode)#Basic_Multilingual_Plane
	//  - non-BMP characters are encoded in UTF 16 by using a surrogate pair of 16-bit codes
	//	  EncodeRune returns 0xfffd if the rune does not need special encoding
	//  - the above RFC provides the info that BMPStrings are NULL terminated.

	ret := make([]byte, 0, 2*len(s)+2)

	for _, r := range s {
		if t, _ := utf16.EncodeRune(r); t != 0xfffd {
			return nil, errors.New("pkcs12: string contains characters that cannot be encoded in UCS-2")
		}
		ret = append(ret, byte(r/256), byte(r%256))
	}

	return append(ret, 0, 0), nil
}