We will define a new 'STU' choice element on the CanonicalResource "versionAlgorithm [x] " with a type of Coding|string Definition: Indicates the mechanism used to compare versions to determine which is more current. Comment: If set as a string, this is a FHIRPath expression that has two additional context variables passed in - %version1 and %version2 and will return a negative number if version1 is newer, a positive number if version2 and a 0 if the version ordering can't be successfully be determined. Rationale: Needed to allow computable resolution of non-versioned references (which are supposed to refer the 'most recent' version) Initial set of codes (extensible binding): semver: Uses the semantic versioning scheme as defined in rfc??? integer: versions are integers and ordered numerically alpha: Simple alphabetic sort on a case-insensitive and accent-insensitive basis.  (Sorting of different cases or accented versions of a character are indeterminate) date: versions are expressed as an ISO date/time syntax (including syntaxes with only portions of a date) natural: sorted according to the algorithm defined here: http://www.naturalordersort.org/   Add a 'warning' invariant for R5+ if version is present and this element is not