In general for simple extensions, should Extension.value have a cardinality of 1..1 with the optionality of extension handled in the part profile? For example, in the PlanDefinition profile, the receiverAddress and authorSignature extensions have cardinalities of 1..1 and 0..1 respective, but in both Extension profiles .value has a cardinality of 0..1. It seems like this means that the requirement for a receiver address can be met by including the extension but doesn't require that an address actually be provided in the extension (because .value isn't required). Would it make more sense to require .value (1..1) so that if the extension is present, there will be a value?