We will add a pre-condition indicating that the two payers must be able to recognize the each other based on an agreed set of payer identifier scheme.  Will add an open issue noting that work is in progress on standardizing how payer identification will be managed, as well as how the FHIR endpoint for a given payer will be found - i.e. through the use of a registry.  For now, this is left to site-to-site negotiation.