NobbZ: i did this:
```nix
    psi-plus = unstable-nixpkgs.pkgs.psi-plus.overrideAttrs rec {
      version = "1.4.1463";

      src = pkgs.fetchFromGitHub {
        owner = "psi-plus";
        repo = "psi-plus-snapshots";
        rev = version;
        sha256 = "ffffffffffffffffffffffffffffffffffffffffffffffffffff";
      };
    };
```
but got this error i don't fully understand:
> error: attempt to call something which is not a function but a set, at /nix/store/c1iikfgrvf0xzn7bpc70vi20z5gg3i27-source/pkgs/stdenv/gener
ic/make-derivation.nix:339:55