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error: attribute 'foo.bar' 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:6:13:
            5|   };
            6|   foo.bar = y;
             |             ^
            7| }