Mapping from a FHIR Composition to an XDS DocumentEntry

Rene Spronk asked this one on one of the HL7 lists ... basically the question is what is the mapping from XDSDocumentEntry to Composition.  I give the mapping backwards, how do you produce an XDS DocumentEntry from a composition, rather than forwards, but it can be used in either direction.

XDSDocumentEntry	Composition	ClinicalDocument
formatCode	(meta.profile)	ClinicalDocument.templateId
uniqueId	(meta.versionId)
languageCode	(language)	ClinicalDocument.languageCode
	identifier	.setId
	status	n/a
classCode (mapped from type), typeCode	type	.code
	class	n/a
patientId, sourcePatientId, sourcePatientInfo	subject	.recordTarget
healthcareFacilityType, practiceSettingCode via reference to encounter/location/physicalType and encounter/location/type	encounter	.componentOf.encompassingEncounter
creationTime	date	.effectiveTime
authorInstitution, authorSpecialty, authorPerson, authorRole via reference to author resource	author	.author.assignedAuthor
title	title	.title
confidentialityCode	confidentiality	.confidentialityCode
	attester	.authenticator/.legalAuthenticator
	mode	implied by .authenticator/.legalAuthenticator
	time	.authenticator.time
legalAuthenticator	party	.authenticator.assignedEnttty
	custodian	.custodian.assignedCustodian
	relatesTo	.relatedDocument
parentDocumentRelationship	code	.relatedDocument.typeCode
parentDocumentId	target[x]	.relatedDocument.id
	event	.documentationOf.serviceEvent
eventCodeList	code	.code
	period	.effectiveTime
serviceStartTime	low
serviceStopTime	high