{ sources ? import ./sources.nix, system ? builtins.currentSystem
, crossSystem ? null, config ? { } }:

let
  # our own overlays:
  local-overlays = [ ];

  globals = if builtins.pathExists ../globals.nix then
    [ (import ../globals.nix) ]
  else
    builtins.trace "globals.nix missing, please add symlink" [ ];

  # merge upstream sources with our own:
  upstream-overlays = [
    (_: super: {

      sources = (super.sources or { }) // sources;
    })
  ];

  overlays = local-overlays ++ globals ++ upstream-overlays;
in import sources.nixpkgs { inherit overlays system crossSystem config; }