1: { callPackage, hello, isAttrSet }:
2:
3: assert isAttrSet { };
4: assert isAttrSet { x = "y"; };
5: assert !(isAttrSet hello);
6: assert !(isAttrSet (callPackage ({ }: x: x) { }));
7: assert !(isAttrSet [ ]);
8: assert !(isAttrSet null);
9: { }