The definition for the extension http://hl7.org/fhir/StructureDefinition/valueset-expression reads:

"An expression that provides an alternative definition of the content of the value set (compose). There are two different ways to use this expression extension: If both an expression (this extension) and a compose element is present, the compose is understood to the make the same statement as the expression. If there is no compose, the expression is the only definition of the value set, and the value set can only be processed by a server that understands the expression syntax, the expression is computable"

it sort of falls apart right at the end, and I'm not sure what to make of that. Is there an example out there of this in use? a terminology server that supports this?