let
  lib = import <nixpkgs/lib>;
in rec {
  inherit lib;

  input = [
    {
      x = ["pool" "zfs"];
      y = ["mdadm" "raid1"];
    }
    {
      x = ["pool" "zfs"];
      y = ["disk" "sda"];
    }
    {
      x = ["mdadm" "raid1"];
      y = ["disk" "sdb"];
    }
    {
      x = ["mdadm" "raid1"];
      y = ["disk" "sdc"];
    }
  ];

  outNodes = node: graph:
    lib.unique
    (builtins.map (e: e.y)
      (builtins.filter (v: v.x == node) graph));

  vertices = graph:
    lib.unique
    (builtins.map (x: x.y) graph ++ builtins.map (x: x.x) graph);

  deleteVertex = node: graph: (builtins.filter (v: v.x != node && v.y != node) graph);

  findSink = graph:
    lib.findFirst
    (v: outNodes v graph == [])
    (lib.trace graph (builtins.abort "No sink found"))
    (vertices graph);

  topSort = graph:
    if graph == []
    then []
    else if builtins.length graph == 1
    then let only = builtins.head graph; in [only.y only.x]
    else let sink = findSink graph; in [sink] ++ topSort (deleteVertex sink graph);

  output = topSort input;
}