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error: attribute 'foo' cannot be merged, because one set is marked as recursive and the other isn't. Use --extra-deprecated-features rec-set-merges to disable this error and make the expression parse as-is with implementation-defined semantics.
       at /pwd/in.nix:6:9:
            5|   };
            6|   foo = rec {
             |         ^
            7|     # merging this attrset to foo.bar, discards the `rec` modifier