Based on discussions at https://chat.fhir.org/#narrow/stream/179266-fhirpath/topic/should.20all.20functions.20work.20on.20repeating.20fields.3F each FHIRPath function should define their behavior for input collections with more than one element.

The FHIRPath functions are pretty good about this, but the functions defined in the FHIR spec are a bit more spotty.

Some of the functions, like "resolve()" cover this case well with language like "For each item in the collection, ...".

But some of the others seem to leave this undefined. For the most part, I think that:

1. functions currently defined in FHIR spec that return a collection should work against an input collection with multiple items; and
2. functions currently defined in FHIR spec that return a single item should only work against an input collection with 

However, that should be made explicit.

Here is the list that I think is underspecified but for which an input collection with more than 1 item should be an error:

• hasValue
• getValue
• memberOf
• subsumes
• subsumedBy
• htmlChecks

The only function that I think is underspecified but for which an input collection with more than 1 item should work is 'ofType'.

https://chat.fhir.org/#narrow/stream/179175-argonaut/topic/Conditions.20warning.20constraint.20bug