A lot of objects can be ungexp-ed into a gexp to substitute in its store path.
``s are one of them:
```scheme
(define WSRT_Measures
(origin
(method url-fetch)
(uri "[...not sure I can disclose...]/WSRT_Measures.ztar")
(sha256
(base32 "1d9qlcmkpsnw8wm7sidp17iwd2d771nsgjwwp568886g5f
Hi,
not really sure if this is, what you asked for, but I solved it by
defining an additional package (let's call it "extension package"
or "wsrt-measures") for the extension file(s), similar to what you
already do. I would unpack the data in the extension package
already. Then I add that ext
