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: identifier.go
    path of file: ./repos/buildkit/vendor/github.com/spdx/tools-golang/spdx/v2/common
    
    the code of the file until where you have to start completion: // SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later

package common

import (
	"encoding/json"
	"fmt"
	"strings"
)

const (
	spdxRefPrefix     = "SPDXRef-"
	documentRefPrefix = "DocumentRef-"
)

// ElementID represents the identifier string portion of an SPDX element
// identifier. DocElementID should be used for any attributes which can
// contain identifiers defined in a diff
    
func (d *DocElementID) UnmarshalJSON(data []byte) (err error) {
	// SPDX identifier will simply be a string
	idStr := string(data)
	idStr = strings.Trim(idStr, "\"")

	// handle special cases
	if idStr == "NONE" || idStr == "NOASSERTION" {
		d.SpecialID = idStr
		return nil
	}

	var idFields []string
	// handle DocumentRef- if present
	if strings.HasPrefix(idStr, documentRefPrefix) {
		// strip out the "DocumentRef-" so we can get the value
		idFields = strings.SplitN(idStr, documentRefPrefix, 2)
		idStr = idFields[1]

		// an SPDXRef can appear after a DocumentRef, separated by a colon
		idFields = strings.SplitN(idStr, ":", 2)
		d.DocumentRefID = idFields[0]

		if len(idFields) == 2 {
			idStr = idFields[1]
		} else {
			return nil
		}
	}

	d.ElementRefID, err = trimElementIdPrefix(idStr)
	return err
}