Implicit Value Set Definition is Confusing

Change existing text / entire section to this:   Instead of a ValueSet resource as the definition, implicit value sets are defined in a specification which references the underlying code system structures and includes a prescribed URI pattern to identify the value set .  HL7 has defined implicit value set URI patterns for some key code systems. Implicit value sets allow the URI to serve as the basis for ValueSet operations such as $expand and $validate-code without the need to create a defining ValueSet resource instance. Some advantages of using implicit value sets are that they may be used: With read-only terminology servers  because a ValueSet resource cannot be posted With HTTP GET requests because they are cacheable To enable text searches (google like) for concepts across the entirety of large code systems (e.g., SNOMED CT) To provide a convenient way to reference all or parts of code systems in the definition of a ValueSet or ConceptMap  In some cases it is not possible to express a value set definition using ValueSet.compose - but it may be possible to express the value set definition with an implicit value set URI. An example of this scenario is all SNOMED CT concept IDs that identify reference sets. Implementers SHOULD NOT create ValueSet resources where the ValueSet.url value matches the pattern of a known implicit value set.  Implicit value set URIs can be used anywhere a value set URI can be used. Support for implicit value set URI patterns varies across terminology servers. Some (but not all) code systems have implicit value set URI patterns defined by HL7 and documented for use with FHIR terminology services. Code system publishers may also define implicit value set URI patterns. FHIR terminology servers might or might not support any or all of these URI patterns. Caution should be exercised when using value set URI patterns that have not been defined by HL7 or the code system publisher.