```
nix-repl> p = import (builtins.fetchTarball https://github.com/nix-community/emacs-overlay/archive/master.tar.gz)
nix-repl> ps = import <nixpkgs> { overlays = [ p ]; }
nix-repl> builtins.hasAttr "emacsWithPackagesFromUsePackage" ps
true
```