Primitive type 'uid'
A unique identifier string is a character string which identifies an object in a globally unique and timeless manner. The allowable formats and values and procedures of this data type are strictly controlled by HL7. At this time, user-assigned identifiers may be certain character representations of ISO Object Identifiers (OID) and DCE Universally Unique Identifiers (UUID). HL7 also reserves the right to assign other forms of UIDs (RUID), such as mnemonic identifiers for code systems.
Note that FHIR strings SHALL NOT exceed 1,048,576 (1024*1024) characters in size
All FHIR elements must have a @value or children
hasValue() or (children().count() > id.count())
Must conform to OID (#.#.#), UUID (32 hexadecimal digits separated by hyphens), or RUID (a letter followed by any combination of letters, numbers or hyphens)
unique id for the element within a resource (for internal references)
Must have either extensions or value[x], not both