nixpkgs overlays must be a list

[mstone]

pkgs/top-level/stage.nix#L208 will always fail if overlays is not a list but this fact is not made adequately clear in the documentation for how to define overlays in the $HOME/.config/nixpkgs/overlays.nix overlays file at doc/overlays.xml#L96. As a result,

a) perhaps pkgs/top-level/impure.nix#L47 can be adjusted to to give a more meaningful error message if the result of importing the overlays file is not a list and/or

b) perhaps doc/overlays.xml#L96 can be augmented with an example to demonstrate that the nixexpr written into the $HOME/.config/nixpkgs/overlays.nix file must evaluate to a list?

Thanks!

[stale]

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