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: license.go
    path of file: ./repos/syft/syft/pkg
    
    the code of the file until where you have to start completion: package pkg

import (
	"fmt"
	"sort"

	"github.com/scylladb/go-set/strset"

	"github.com/anchore/syft/internal/log"
	"github.com/anchore/syft/syft/artifact"
	"github.com/anchore/syft/syft/file"
	"github.com/anchore/syft/syft/license"
)

var _ sort.Interface = (*Licenses)(nil)

// License represents an SPDX Expression or license value extracted from a packages metadata
// We want to ignore URLs and Location since we merge these fields across equal licenses.
// A License is a unique combination of value, expression and type, where
// its sources are always considered merged and additions to the evidence
// of where it was found and how it was sourced.
// This is different from how we treat a package since we consider package paths
// in order to distinguish if packages should be kept separate
// this is differ
    
func (s License) Merge(l License) (*License, error) {
	sHash, err := artifact.IDByHash(s)
	if err != nil {
		return nil, err
	}
	lHash, err := artifact.IDByHash(l)
	if err != nil {
		return nil, err
	}
	if err != nil {
		return nil, err
	}
	if sHash != lHash {
		return nil, fmt.Errorf("cannot merge licenses with different hash")
	}

	// try to keep s.URLs unallocated unless necessary (which is the default state from the constructor)
	if len(l.URLs) > 0 {
		s.URLs = append(s.URLs, l.URLs...)
	}

	if len(s.URLs) > 0 {
		s.URLs = strset.New(s.URLs...).List()
		sort.Strings(s.URLs)
	}

	if l.Locations.Empty() {
		return &s, nil
	}

	// since the set instance has a reference type (map) we must make a new instance
	locations := file.NewLocationSet(s.Locations.ToSlice()...)
	locations.Add(l.Locations.ToSlice()...)
	s.Locations = locations

	return &s, nil
}