It could be useful to have a lib.fileset.empty, a file set containing no files, maybe for

• Third-party functions returning file sets like
• A default value when folding over a list of no elements in user code

Though I haven't really encountered a good use case for it, we might not need it.
However it would be trivial to add, all the code is already there, just needs to be exposed.

If somebody needs this, please 👍 and explain your use case.

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