{
  description = "my project description";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-23.05";
    flake-utils.url = "github:numtide/flake-utils";
    colmena.url = "github:zhaofengli/colmena";

    nixos-hardware.url = "github:NixOS/nixos-hardware/master";
  };

  outputs = inputs@{self, nixpkgs, flake-utils, ...}:
  flake-utils.lib.eachDefaultSystem (system:
    let
    # nixpkgs for native system.
    pkgs = import nixpkgs {
      inherit system;
      overlays = [
        inputs.colmena.overlay
      ];
    };
    # Native VM config.
    riversong = nixpkgs.lib.nixosSystem {
      inherit pkgs;
      modules = [
        ./citrus.nix
        "${nixpkgs}/nixos/modules/virtualisation/qemu-vm.nix"
        ./platforms/riversong.nix
      ];
    };
    in rec {
      # Install colmena in dev shell for deployment.
      devShell = pkgs.mkShell {
        packages = with pkgs; [ colmena ];
      };
      # Run testing VM using `nix run`
      packages.default = riversong.config.system.build.vm;
    }) // (let
      # Target device (RPi4 aarch64) nixpkgs.
      pkgs = import nixpkgs {
        system = "aarch64-linux";
        overlays = [
          (final: super: {
            makeModulesClosure = x:
            super.makeModulesClosure (x // { allowMissing = true; });
          })
        ];
      };
      # Modules for colmena and sd card image are the same.
      modules = [
        ./citrus.nix
        "${nixpkgs}/nixos/modules/installer/sd-card/sd-image-aarch64.nix"
        ./platforms/melodypond.nix
        inputs.nixos-hardware.nixosModules.raspberry-pi-4
      ];
    in {
      # Colmena deploy manifest
      colmena = {
        meta.nixpkgs = pkgs;
        melodypond = {
          deployment.targetUser = "geekygay";
          imports = modules;
        };
      };
      # SD card image build.
      melodypond = (nixpkgs.lib.nixosSystem {
          inherit pkgs modules;
        }).config.system.build.sdImage;
    });
}