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: document.go
    path of file: ./repos/buildkit/vendor/github.com/spdx/tools-golang/spdx/v2/v2_3
    
    the code of the file until where you have to start completion: // Package v2_3 Package contains the struct definition for an SPDX Document
// and its constituent parts.
// SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
package v2_3

import (
	"encoding/json"
	"fmt"

	converter "github.com/anchore/go-struct-converter"

	"github.com/spdx/tools-golang/spdx/v2/common"
)

const Version = "SPDX-2.3"
const DataLicense = "CC0-1.0"

// ExternalDocumentRef is a reference to an external SPDX document as defined in section 6.6
type ExternalDocumentRef struct {
	// DocumentRefID is the ID string defined in the start of the
	// reference. It should _not_ contain the "DocumentRef-" part
	// of the mandatory ID string.
	DocumentRefID string `json:"externalDocumentId"`

	// URI is the URI defined for the external document
	URI string `json:"spdxDocument"`

	// Checksum is the actual hash data
	Checksum common.Checksum `json:"checksum"`
}

// Document is an SPDX Document:
// See https://spdx.github.io/spdx-spec/v2.3/document-creation-information
type Document struct {
	// 6.1: SPDX Version; should be in the format "SPDX-<version>"
	// Cardinality: mandatory, one
	SPDXVersion string `json:"spdxVersion"`

	// 6.2: D
    
func (d *Document) UnmarshalJSON(b []byte) error {
	type doc Document
	type extras struct {
		DocumentDescribes []common.DocElementID `json:"documentDescribes"`
	}

	var d2 doc
	if err := json.Unmarshal(b, &d2); err != nil {
		return err
	}

	var e extras
	if err := json.Unmarshal(b, &e); err != nil {
		return err
	}

	*d = Document(d2)

	relationshipExists := map[string]bool{}
	serializeRel := func(r *Relationship) string {
		refA := r.RefA
		refB := r.RefB
		rel := r.Relationship

		// we need to serialize the opposite for CONTAINED_BY and DESCRIBED_BY
		// so that it will match when we try to de-duplicate during deserialization.
		switch r.Relationship {
		case common.TypeRelationshipContainedBy:
			rel = common.TypeRelationshipContains
			refA = r.RefB
			refB = r.RefA
		case common.TypeRelationshipDescribeBy:
			rel = common.TypeRelationshipDescribe
			refA = r.RefB
			refB = r.RefA
		}
		return fmt.Sprintf("%v-%v->%v", common.RenderDocElementID(refA), rel, common.RenderDocElementID(refB))
	}

	// index current list of relationships to ensure no duplication
	for _, r := range d.Relationships {
		relationshipExists[serializeRel(r)] = true
	}

	// build relationships for documentDescribes field
	for _, id := range e.DocumentDescribes {
		r := &Relationship{
			RefA: common.DocElementID{
				ElementRefID: d.SPDXIdentifier,
			},
			RefB:         id,
			Relationship: common.TypeRelationshipDescribe,
		}

		if !relationshipExists[serializeRel(r)] {
			d.Relationships = append(d.Relationships, r)
			relationshipExists[serializeRel(r)] = true
		}
	}

	// build relationships for package hasFiles field
	// build relationships for package hasFiles field
	for _, p := range d.Packages {
		for _, f := range p.hasFiles {
			r := &Relationship{
				RefA: common.DocElementID{
					ElementRefID: p.PackageSPDXIdentifier,
				},
				RefB:         f,
				Relationship: common.TypeRelationshipContains,
			}
			if !relationshipExists[serializeRel(r)] {
				d.Relationships = append(d.Relationships, r)
				relationshipExists[serializeRel(r)] = true
			}
		}

		p.hasFiles = nil
	}

	return nil
}