a.nix

  nixpkgs.overlays = [
    (import (builtins.fetchTarball {
      url = https://github.com/nix-community/neovim-nightly-overlay/archive/master.tar.gz;
    }))
    (self: super: {
      sbclOverlay = super.sbcl.overrideAttrs (oldAttrs: rec{
        version = "2.1.1";
      });
    })
  ];