#282: Provided rationale for the use of __null__ in the prefetch response.

Author: brynrhodes

docs/specification/1.0.md

@@ -234,7 +234,9 @@ The resulting response, which MUST be rendered in a single page — no "next pag
 
 > Note that the reason prefetch results are not allowed to include next page links is that if the prefetched data contains just a single page of data, the CDS Service has no means to retrieve the subsequent pages of data. Consider, for example, a CDS Hooks implementation that does not expose a FHIR server.
 
-The CDS Service MUST NOT receive any prefetch template key that the EHR chooses not to satisfy; in which case the prefetch template key SHOULD NOT be sent. Similarly, if the EHR encounters an error while prefetching any data, the prefetch template key SHOULD NOT be sent to the CDS Service. If the EHR has no data to populate a template prefetch key, the prefetch template key MUST have a value of __null__. It is the CDS Service's responsibility to check prefetched data against its template to determine what requests were satisfied (if any) and to manually retrieve any additional necessary data. If the CDS Service is unable to obtain required data because it cannot access the FHIR server and the request did not contain the necessary prefetch keys, the service SHALL respond with an HTTP 412 Precondition Failed status code.
+The CDS Service MUST NOT receive any prefetch template key that the EHR chooses not to satisfy; in which case the prefetch template key SHOULD NOT be sent. Similarly, if the EHR encounters an error while prefetching any data, the prefetch template key SHOULD NOT be sent to the CDS Service. If the EHR has no data to populate a template prefetch key, the prefetch template key MUST have a value of __null__. Note that the __null__ result is used rather than a bundle with zero entries to account for the possibility that the prefetch url is a single-resource request.
+
+It is the CDS Service's responsibility to check prefetched data against its template to determine what requests were satisfied (if any) and to manually retrieve any additional necessary data. If the CDS Service is unable to obtain required data because it cannot access the FHIR server and the request did not contain the necessary prefetch keys, the service SHALL respond with an HTTP 412 Precondition Failed status code.
 
 #### Example prefetch templates