FHIR Shorthand does not need to be extended to support overriding the url of an extension. FSH already supports this for extensions, profiles, and any other canonical resources. Since caret paths allow authors to modify the underlying elements of the extension's StructureDefinition, setting the url is done via a simple assignment:   * ^url = "http: //example.org/custom/extension/url/1 We will add text to section 3.5.4 to highlight this feature:   Extension metadata that does not have a dedicated keyword may be specified using assignment rules with caret paths (e.g., `^url`, `^status`, `^purpose`). We will also add similar language to the sections on defining profiles, resources, logical models, value sets, and code systems.