The example in https://hl7.org/fhirpath/#unionother-collection introduces collections "A" and "B" (in uppercase, which is odd), and then uses them in an expression as "a" and "b", without providing information about where "a" and "b" are coming from. This caused some confusion-- see https://chat.fhir.org/#narrow/stream/179266-fhirpath/topic/Context.20for.20.60.2Eunion.60. In that thread, a more clear example was given, namely that for a Patient resource as the context, ```name.select(use.union(given))``` is the same as ```name.select(use | given)```. I suggest changing the example to that one.