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: ocsp.go
    path of file: ./repos/scan4all/vendor/golang.org/x/crypto/ocsp
    
    the code of the file until where you have to start completion: // Copyright 2013 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 ocsp parses OCSP responses as specified in RFC 2560. OCSP responses
// are signed messages attesting to the validity of a certificate for a small
// period of time. This is used to manage revocation for X.509 certificates.
package ocsp // import "golang.org/x/crypto/ocsp"

import (
	"crypto"
	"crypto
    
func (req *Request) Marshal() ([]byte, error) {
	hashAlg := getOIDFromHashAlgorithm(req.HashAlgorithm)
	if hashAlg == nil {
		return nil, errors.New("Unknown hash algorithm")
	}
	return asn1.Marshal(ocspRequest{
		tbsRequest{
			Version: 0,
			RequestList: []request{
				{
					Cert: certID{
						pkix.AlgorithmIdentifier{
							Algorithm:  hashAlg,
							Parameters: asn1.RawValue{Tag: 5 /* ASN.1 NULL */},
						},
						req.IssuerNameHash,
						req.IssuerKeyHash,
						req.SerialNumber,
					},
				},
			},
		},
	})
}