diff --git a/flake.nix b/flake.nix
new file mode 100644
index 00000000..ed2b35f1
--- /dev/null
+++ b/flake.nix
@@ -0,0 +1,514 @@
+{
+  description = "xo-unit: c++ compile-time dimension checking and unit conversion";
+
+  # Adapted from xo-nix2/flake.nix
+
+  # MANIFESTO
+  # No build instructions in flake.nix
+  # - Following Jade Lovelace's advice
+  # - Build instructions are in pkgs/*.nix
+  # - Each pkgs/*.nix is intended to work 'like a .nix file in nixpkgs'
+  #   I'm being lazy about source hashes,  since flake.nix supplies them.
+  #
+  # Motivation (per JL) versus doing everything in flake.nix:
+  # - nixpkgs-ready
+  # - parameterized
+  # - overridable
+  # - still works if cross-compiling
+  #
+  # Instead:  using flake.nix as entry point:
+  # - pin nixpkgs to a specific revision,  for reproducibility
+  # - pin our candidate packages (pkgs/*.nix),  for the same reason.
+
+  # to determine specific hash for nixpkgs:
+  # 1. $ cd ~/proj/nixpkgs
+  # 2. $ git checkout release-23.05
+  # 3. $ git fetch
+  # 4. $ git pull
+  # 5. $ git log -1
+  #    take this hash,  then substitue for ${hash} in:
+  #      inputs.nixpkgs.url = "https://github.com/NixOS/nixpkgs/archive/${hash}.tar.gz";
+  #    below
+  #inputs.nixpkgs.url = "https://github.com/NixOS/nixpkgs/archive/9a333eaa80901efe01df07eade2c16d183761fa3.tar.gz";
+
+  # as sbove but instead of {release-23.05} use {release-23.11}
+  #   gcc -> 12.3.0
+  #   python -> 3.11.6
+  #
+  inputs.nixpkgs.url = "https://github.com/NixOS/nixpkgs/archive/217b3e910660fbf603b0995a6d2c3992aef4cc37.tar.gz"; # asof 10mar2024
+  #inputs.nixpkgs.url = "https://github.com/NixOS/nixpkgs/archive/4dd376f7943c64b522224a548d9cab5627b4d9d6.tar.gz";
+
+  # inputs.nixpkgs.url
+  #   = "https://github.com/NixOS/nixpkgs/archive/fac3684647cc9d6dfb2a39f3f4b7cf5fc89c96b6.tar.gz"; # asof 8feb2024
+  # fac3684647.. asof 17oct2023
+  # instead of
+  #   inputs.nixpkgs.url = "github:nixos/nixpkgs/23.05";
+
+  inputs.flake-utils.url = "github:numtide/flake-utils";
+
+  # To add a new package,  visit placeholder-A .. placeholder-E
+
+  inputs.xo-cmake-path          = { type = "github"; owner = "Rconybea"; repo = "xo-cmake";          flake = false; };
+  inputs.xo-indentlog-path      = { type = "github"; owner = "Rconybea"; repo = "indentlog";         flake = false; };
+  inputs.xo-refcnt-path         = { type = "github"; owner = "Rconybea"; repo = "refcnt";            flake = false; };
+  inputs.xo-subsys-path         = { type = "github"; owner = "Rconybea"; repo = "subsys";            flake = false; };
+  inputs.xo-randomgen-path      = { type = "github"; owner = "Rconybea"; repo = "randomgen";         flake = false; };
+  inputs.xo-ordinaltree-path    = { type = "github"; owner = "Rconybea"; repo = "xo-ordinaltree";    flake = false; };
+  inputs.xo-pyutil-path         = { type = "github"; owner = "Rconybea"; repo = "xo-pyutil";         flake = false; };
+  inputs.xo-reflect-path        = { type = "github"; owner = "Rconybea"; repo = "reflect";           flake = false; };
+  inputs.xo-pyreflect-path      = { type = "github"; owner = "Rconybea"; repo = "xo-pyreflect";      flake = false; };
+  inputs.xo-printjson-path      = { type = "github"; owner = "Rconybea"; repo = "xo-printjson";      flake = false; };
+  inputs.xo-pyprintjson-path    = { type = "github"; owner = "Rconybea"; repo = "xo-pyprintjson";    flake = false; };
+  inputs.xo-callback-path       = { type = "github"; owner = "Rconybea"; repo = "xo-callback";       flake = false; };
+  inputs.xo-webutil-path        = { type = "github"; owner = "Rconybea"; repo = "xo-webutil";        flake = false; };
+  inputs.xo-pywebutil-path      = { type = "github"; owner = "Rconybea"; repo = "xo-pywebutil";      flake = false; };
+  inputs.xo-reactor-path        = { type = "github"; owner = "Rconybea"; repo = "xo-reactor";        flake = false; };
+  inputs.xo-pyreactor-path      = { type = "github"; owner = "Rconybea"; repo = "xo-pyreactor";      flake = false; };
+  inputs.xo-simulator-path      = { type = "github"; owner = "Rconybea"; repo = "xo-simulator";      flake = false; };
+  inputs.xo-pysimulator-path    = { type = "github"; owner = "Rconybea"; repo = "xo-pysimulator";    flake = false; };
+  inputs.xo-distribution-path   = { type = "github"; owner = "Rconybea"; repo = "xo-distribution";   flake = false; };
+  inputs.xo-pydistribution-path = { type = "github"; owner = "Rconybea"; repo = "xo-pydistribution"; flake = false; };
+  inputs.xo-process-path        = { type = "github"; owner = "Rconybea"; repo = "xo-process";        flake = false; };
+  inputs.xo-pyprocess-path      = { type = "github"; owner = "Rconybea"; repo = "xo-pyprocess";      flake = false; };
+  inputs.xo-statistics-path     = { type = "github"; owner = "Rconybea"; repo = "xo-statistics";     flake = false; };
+  inputs.xo-kalmanfilter-path   = { type = "github"; owner = "Rconybea"; repo = "xo-kalmanfilter";   flake = false; };
+  inputs.xo-pykalmanfilter-path = { type = "github"; owner = "Rconybea"; repo = "xo-pykalmanfilter"; flake = false; };
+  inputs.xo-websock-path        = { type = "github"; owner = "Rconybea"; repo = "xo-websock";        flake = false; };
+  inputs.xo-pywebsock-path      = { type = "github"; owner = "Rconybea"; repo = "xo-pywebsock";      flake = false; };
+  # placeholder-A
+
+  outputs
+  = { self,
+      nixpkgs,
+      flake-utils,
+      xo-cmake-path,
+      xo-indentlog-path,
+      xo-refcnt-path,
+      xo-subsys-path,
+      xo-reflect-path,
+      xo-pyreflect-path,
+      xo-randomgen-path,
+      xo-ordinaltree-path,
+      xo-pyutil-path,
+      xo-printjson-path,
+      xo-pyprintjson-path,
+      xo-callback-path,
+      xo-webutil-path,
+      xo-pywebutil-path,
+      xo-reactor-path,
+      xo-pyreactor-path,
+      xo-simulator-path,
+      xo-pysimulator-path,
+      xo-distribution-path,
+      xo-pydistribution-path,
+      xo-process-path,
+      xo-pyprocess-path,
+      xo-statistics-path,
+      xo-kalmanfilter-path,
+      xo-pykalmanfilter-path,
+      xo-websock-path,
+      xo-pywebsock-path,
+      # placeholder-B
+    } :
+      # out :: system -> {packages, devShells}
+      let
+        out
+          = system :
+            let
+              pkgs = nixpkgs.legacyPackages.${system};
+
+              # could try using
+              #   appliedOverlay = (pkgs.extend self.overlays.default)
+              # but it doesn't seem to work the way I expect,
+              # For example, wants to pickup 2.7.11 python for xo-pyutil !
+              #
+              appliedOverlay = self.overlays.default pkgs pkgs;
+
+            in
+              {
+                #xo-cmake-dir = "${self.packages.${system}.xo-cmake}/share/cmake";
+
+                # reminder:
+                # 'packages' comprises the output of this flake;
+                # each defn invokes a build
+                #   ./pkgs/$example.nix
+                # using
+                #   cmake-examples-$example-path
+                # above for source code
+
+                packages.xo-cmake = appliedOverlay.xo-cmake;
+                packages.xo-indentlog = appliedOverlay.xo-indentlog;
+                packages.xo-refcnt = appliedOverlay.xo-refcnt;
+                packages.xo-subsys = appliedOverlay.xo-subsys;
+                packages.xo-randomgen = appliedOverlay.xo-randomgen;
+                packages.xo-ordinaltree = appliedOverlay.xo-ordinaltree;
+                packages.xo-pyutil = appliedOverlay.xo-pyutil;
+                packages.xo-reflect = appliedOverlay.xo-reflect;
+                packages.xo-pyreflect = appliedOverlay.xo-pyreflect;
+                packages.xo-printjson = appliedOverlay.xo-printjson;
+                packages.xo-pyprintjson = appliedOverlay.xo-pyprintjson;
+                packages.xo-callback = appliedOverlay.xo-callback;
+                packages.xo-webutil = appliedOverlay.xo-webutil;
+                packages.xo-pywebutil = appliedOverlay.xo-pywebutil;
+                packages.xo-reactor = appliedOverlay.xo-reactor;
+                packages.xo-pyreactor = appliedOverlay.xo-pyreactor;
+                packages.xo-simulator = appliedOverlay.xo-simulator;
+                packages.xo-pysimulator = appliedOverlay.xo-pysimulator;
+                packages.xo-distribution = appliedOverlay.xo-distribution;
+                packages.xo-pydistribution = appliedOverlay.xo-pydistribution;
+                packages.xo-process = appliedOverlay.xo-process;
+                packages.xo-pyprocess = appliedOverlay.xo-pyprocess;
+                packages.xo-statistics = appliedOverlay.xo-statistics;
+                packages.xo-kalmanfilter = appliedOverlay.xo-kalmanfilter;
+                packages.xo-pykalmanfilter = appliedOverlay.xo-pykalmanfilter;
+                packages.xo-websock = appliedOverlay.xo-websock;
+                packages.xo-pywebsock = appliedOverlay.xo-pywebsock;
+                # placeholder-C
+
+                packages.xo-userenv = appliedOverlay.xo-userenv;
+
+                devShells = appliedOverlay.devShells;
+              };
+      in
+        flake-utils.lib.eachDefaultSystem
+          out
+        //
+        {
+          # introduce overlay to extend nixpkgs with our local packages,
+          # (which ofc are not present in nixpkgs,  though same form would work if they were present)
+          #
+          overlays.default = final: prev:
+            (
+              let
+                # can use
+                #  $ nix-env -qaP | grep \.boost            # show known boost versions
+                #  $ nix-env -qaP | grep \.python.*Packages # show known python versions
+
+                stdenv = prev.stdenv;
+
+                boost = prev.boost182;
+                python = prev.python311Full;
+                pythonPackages = prev.python311Packages;
+                #doxygen = prev.doxygen;
+
+                pybind11 = pythonPackages.pybind11;
+                #breathe = python3Packages.breathe;
+                #sphinx = python3Packages.sphinx;
+                #sphinx-rtd-theme = python3Packages.sphinx-rtd-theme;
+
+                #extras1 = { boost = boost; };
+                #extras2 = { boost = boost; python3Packages = python3Packages; pybind11 = pybind11; };
+                #extras3 = { boost = boost; python3Packages = python3Packages; pybind11 = pybind11; doxygen = doxygen; };
+                #extras4 = extras3 // { breathe = breathe; };
+
+                xo-cmake =
+                  (prev.callPackage ./pkgs/xo-cmake.nix {}).overrideAttrs
+                    (old: { src = xo-cmake-path; });
+
+                xo-indentlog =
+                  (prev.callPackage ./pkgs/xo-indentlog.nix { xo-cmake = xo-cmake; }).overrideAttrs
+                    (old: { src = xo-indentlog-path; });
+
+                xo-subsys =
+                  (prev.callPackage ./pkgs/xo-subsys.nix { xo-cmake = xo-cmake; }).overrideAttrs
+                    (old: { src = xo-subsys-path; });
+
+                xo-refcnt =
+                  (prev.callPackage ./pkgs/xo-refcnt.nix { xo-cmake = xo-cmake;
+                                                           xo-indentlog = xo-indentlog; }).overrideAttrs
+                    (old: { src = xo-refcnt-path; });
+
+                xo-randomgen =
+                  (prev.callPackage ./pkgs/xo-randomgen.nix { xo-cmake = xo-cmake;
+                                                              xo-indentlog = xo-indentlog; }).overrideAttrs
+                    (old: { src = xo-randomgen-path; });
+
+                xo-ordinaltree =
+                  (prev.callPackage ./pkgs/xo-ordinaltree.nix { xo-cmake = xo-cmake;
+                                                                xo-refcnt = xo-refcnt;
+                                                                xo-randomgen = xo-randomgen; }).overrideAttrs
+                    (old: { src = xo-ordinaltree-path; });
+
+                xo-pyutil =
+                  (prev.callPackage ./pkgs/xo-pyutil.nix { xo-cmake = xo-cmake;
+                                                           xo-refcnt = xo-refcnt;
+                                                           python = python;
+                                                           pybind11 = pybind11;
+                                                         }).overrideAttrs
+                    (old: { src = xo-pyutil-path; });
+
+                xo-reflect =
+                  (prev.callPackage ./pkgs/xo-reflect.nix { xo-cmake = xo-cmake;
+                                                            xo-subsys = xo-subsys;
+                                                            xo-refcnt = xo-refcnt; }).overrideAttrs
+                    (old: { src = xo-reflect-path; });
+
+                xo-pyreflect =
+                  (prev.callPackage ./pkgs/xo-pyreflect.nix { xo-cmake = xo-cmake;
+                                                              xo-refcnt = xo-refcnt;
+                                                              xo-pyutil = xo-pyutil;
+                                                              xo-reflect = xo-reflect; }).overrideAttrs
+                    (old: { src = xo-pyreflect-path; });
+
+                xo-unit =
+                  (prev.callPackage ./pkgs/xo-unit.nix { xo-cmake = xo-cmake;
+                                                         xo-reflect = xo-reflect; }).overrideAttrs
+                    (old: { src = ./.; });
+
+                xo-printjson =
+                  (prev.callPackage ./pkgs/xo-printjson.nix { xo-cmake = xo-cmake;
+                                                              xo-reflect = xo-reflect; }).overrideAttrs
+                    (old: { src = xo-printjson-path; });
+
+                xo-pyprintjson =
+                  (prev.callPackage ./pkgs/xo-pyprintjson.nix { xo-cmake = xo-cmake;
+                                                                xo-pyutil = xo-pyutil;
+                                                                xo-printjson = xo-printjson;
+                                                                xo-pyreflect = xo-pyreflect; }).overrideAttrs
+                    (old: { src = xo-pyprintjson-path; });
+
+                xo-callback =
+                  (prev.callPackage ./pkgs/xo-callback.nix { xo-cmake = xo-cmake;
+                                                             xo-reflect = xo-reflect; }).overrideAttrs
+                    (old: { src = xo-callback-path; });
+
+                xo-webutil =
+                  (prev.callPackage ./pkgs/xo-webutil.nix { xo-cmake = xo-cmake;
+                                                            xo-reflect = xo-reflect;
+                                                            xo-callback = xo-callback; }).overrideAttrs
+                    (old: { src = xo-webutil-path; });
+
+                xo-pywebutil =
+                  (prev.callPackage ./pkgs/xo-pywebutil.nix { xo-cmake = xo-cmake;
+                                                              xo-webutil = xo-webutil;
+                                                              xo-pyutil = xo-pyutil; }).overrideAttrs
+                    (old: { src = xo-pywebutil-path; });
+
+                xo-reactor =
+                  (prev.callPackage ./pkgs/xo-reactor.nix { xo-cmake = xo-cmake;
+                                                            xo-randomgen = xo-randomgen;
+                                                            xo-webutil = xo-webutil;
+                                                            xo-printjson = xo-printjson;
+                                                            xo-ordinaltree = xo-ordinaltree; }).overrideAttrs
+                    (old: { src = xo-reactor-path; });
+
+                xo-pyreactor =
+                  (prev.callPackage ./pkgs/xo-pyreactor.nix { xo-cmake = xo-cmake;
+                                                              xo-reactor = xo-reactor;
+                                                              xo-pyutil = xo-pyutil;
+                                                              xo-pyreflect = xo-pyreflect;
+                                                              xo-pyprintjson = xo-pyprintjson;
+                                                            }).overrideAttrs
+                    (old: { src = xo-pyreactor-path; });
+
+                xo-simulator =
+                  (prev.callPackage ./pkgs/xo-simulator.nix { xo-cmake = xo-cmake;
+                                                              xo-reactor = xo-reactor;
+                                                            }).overrideAttrs
+                    (old: { src = xo-simulator-path; });
+
+                xo-pysimulator =
+                  (prev.callPackage ./pkgs/xo-pysimulator.nix { xo-cmake = xo-cmake;
+                                                                xo-simulator = xo-simulator;
+                                                                xo-pyutil = xo-pyutil;
+                                                                xo-pyreactor = xo-pyreactor;
+                                                            }).overrideAttrs
+                    (old: { src = xo-pysimulator-path; });
+
+                xo-distribution =
+                  (prev.callPackage ./pkgs/xo-distribution.nix { xo-cmake = xo-cmake;
+                                                                 xo-refcnt = xo-refcnt;
+                                                               }).overrideAttrs
+                    (old: { src = xo-distribution-path; });
+
+                xo-pydistribution =
+                  (prev.callPackage ./pkgs/xo-pydistribution.nix { xo-cmake = xo-cmake;
+                                                                   xo-distribution = xo-distribution;
+                                                                   xo-pyutil = xo-pyutil;
+                                                                 }).overrideAttrs
+                    (old: { src = xo-pydistribution-path; });
+
+                xo-process =
+                  (prev.callPackage ./pkgs/xo-process.nix { xo-cmake = xo-cmake;
+                                                            xo-simulator = xo-simulator;
+                                                            xo-randomgen = xo-randomgen;
+                                                          }).overrideAttrs
+                    (old: { src = xo-process-path; });
+
+                xo-pyprocess =
+                  (prev.callPackage ./pkgs/xo-pyprocess.nix { xo-cmake = xo-cmake;
+                                                              xo-process = xo-process;
+                                                              xo-pyutil = xo-pyutil;
+                                                              xo-pywebutil = xo-pywebutil;
+                                                              xo-pyreactor = xo-pyreactor;
+                                                            }).overrideAttrs
+                    (old: { src = xo-pyprocess-path; });
+
+                xo-statistics =
+                  (prev.callPackage ./pkgs/xo-statistics.nix { xo-cmake = xo-cmake;
+                                                               #xo-reactor = xo-reactor;
+                                                               }).overrideAttrs
+                    (old: { src = xo-statistics-path; });
+
+                xo-kalmanfilter =
+                  (prev.callPackage ./pkgs/xo-kalmanfilter.nix { xo-cmake = xo-cmake;
+                                                                 xo-statistics = xo-statistics;
+                                                                 xo-reactor = xo-reactor;
+                                                               }).overrideAttrs
+                    (old: { src = xo-kalmanfilter-path; });
+
+
+                xo-pykalmanfilter =
+                  (prev.callPackage ./pkgs/xo-pykalmanfilter.nix { xo-cmake = xo-cmake;
+                                                                   xo-pyutil = xo-pyutil;
+                                                                   xo-kalmanfilter = xo-kalmanfilter;
+                                                                   xo-pyreactor = xo-pyreactor;
+                                                               }).overrideAttrs
+                    (old: { src = xo-pykalmanfilter-path; });
+
+                xo-websock =
+                  (prev.callPackage ./pkgs/xo-websock.nix { xo-cmake = xo-cmake;
+                                                            xo-reactor = xo-reactor;
+                                                               }).overrideAttrs
+                    (old: { src = xo-websock-path; });
+
+                xo-pywebsock =
+                  (prev.callPackage ./pkgs/xo-pywebsock.nix { xo-cmake = xo-cmake;
+                                                              xo-websock = xo-websock;
+                                                              xo-pyutil = xo-pyutil;
+                                                              xo-pywebutil = xo-pywebutil;
+                                                            }).overrideAttrs
+                    (old: { src = xo-pywebsock-path; });
+
+                # placeholder-D
+
+                # user environment with all xo libraries present
+                xo-userenv =
+                  (prev.callPackage ./pkgs/xo-userenv.nix { xo-cmake = xo-cmake;
+                                                            xo-indentlog = xo-indentlog;
+                                                            xo-callback = xo-callback;
+                                                            xo-subsys = xo-subsys;
+                                                            xo-refcnt = xo-refcnt;
+                                                            xo-randomgen = xo-randomgen;
+                                                            xo-ordinaltree = xo-ordinaltree;
+                                                            xo-pyutil = xo-pyutil;
+                                                            xo-reflect = xo-reflect;
+                                                            xo-pyreflect = xo-pyreflect;
+                                                            xo-unit = xo-unit;
+                                                            xo-printjson = xo-printjson;
+                                                            xo-pyprintjson = xo-pyprintjson;
+                                                            xo-webutil = xo-webutil;
+                                                            xo-pywebutil = xo-pywebutil;
+                                                            xo-reactor = xo-reactor;
+                                                            xo-pyreactor = xo-pyreactor;
+                                                            xo-simulator = xo-simulator;
+                                                            xo-pysimulator = xo-pysimulator;
+                                                            xo-distribution = xo-distribution;
+                                                            xo-pydistribution = xo-pydistribution;
+                                                            xo-process = xo-process;
+                                                            xo-pyprocess = xo-pyprocess;
+                                                            xo-statistics = xo-statistics;
+                                                            xo-kalmanfilter = xo-kalmanfilter;
+                                                            xo-pykalmanfilter = xo-pykalmanfilter;
+                                                            xo-websock = xo-websock;
+                                                            xo-pywebsock = xo-pywebsock;
+                                                          }).overrideAttrs(old: {});
+
+
+              in
+                # attrs in this set provide derivations with all overlay changes applied.
+                #
+                # REMINDER: need expression like
+                #             packages.xo-foo = appliedOverlay.xo-foo;
+                #           above to export
+                {
+                  xo-cmake = xo-cmake;
+                  xo-indentlog = xo-indentlog;
+                  xo-subsys = xo-subsys;
+                  xo-refcnt = xo-refcnt;
+                  xo-randomgen = xo-randomgen;
+                  xo-ordinaltree = xo-ordinaltree;
+                  xo-pyutil = xo-pyutil;
+                  xo-reflect = xo-reflect;
+                  xo-pyreflect = xo-pyreflect;
+                  xo-unit = xo-unit;
+                  xo-printjson = xo-printjson;
+                  xo-pyprintjson = xo-pyprintjson;
+                  xo-callback = xo-callback;
+                  xo-webutil = xo-webutil;
+                  xo-pywebutil = xo-pywebutil;
+                  xo-reactor = xo-reactor;
+                  xo-pyreactor = xo-pyreactor;
+                  xo-simulator = xo-simulator;
+                  xo-pysimulator = xo-pysimulator;
+                  xo-distribution = xo-distribution;
+                  xo-pydistribution = xo-pydistribution;
+                  xo-process = xo-process;
+                  xo-pyprocess = xo-pyprocess;
+                  xo-statistics = xo-statistics;
+                  xo-kalmanfilter = xo-kalmanfilter;
+                  xo-pykalmanfilter = xo-pykalmanfilter;
+                  xo-websock = xo-websock;
+                  xo-pywebsock = xo-pywebsock;
+                  # placeholder-E
+
+                  xo-userenv = xo-userenv;
+
+                  devShells = {
+                    default = prev.mkShell.override
+                      # but may need prev.clang16Stdenv instead of prev.stdenv here on macos
+                      { stdenv = prev.stdenv; }
+
+                      { packages
+                        = [ python
+                            pybind11
+                            pythonPackages.coverage
+                            pythonPackages.sphinx
+                            pythonPackages.sphinx-rtd-theme
+                            pythonPackages.breathe
+                            # pythonPackages.pyarrow
+                            boost   # really for filemerge
+
+                            prev.llvmPackages_16.clang-unwrapped
+
+                            prev.anki
+                            prev.mesa
+                            prev.egl-wayland
+
+                            prev.emacs29
+                            prev.notmuch
+                            prev.emacsPackages.notmuch
+                            prev.inconsolata-lgc
+
+                            prev.doxygen
+                            prev.graphviz
+
+                            prev.ditaa
+                            prev.semgrep
+                            prev.ripgrep
+                            prev.git
+                            prev.openssh
+                            prev.cmake
+                            prev.gdb
+                            prev.which
+                            prev.man
+                            prev.man-pages
+                            prev.less
+                            prev.tree
+                            prev.nix-tree
+                            prev.lcov
+
+                            prev.arrow-cpp
+                            prev.libwebsockets
+                            prev.jsoncpp
+                            prev.eigen
+                            prev.catch2
+                            prev.pkg-config
+                            prev.zlib
+                          ];
+                      };
+                  };
+                });
+        };
+
+}
diff --git a/pkgs/xo-unit.nix b/pkgs/xo-unit.nix
new file mode 100644
index 00000000..093c3091
--- /dev/null
+++ b/pkgs/xo-unit.nix
@@ -0,0 +1,37 @@
+{
+  # nixpkgs dependencies
+  stdenv, cmake, catch2, # ... other deps here
+
+  # xo dependencies
+  xo-cmake, xo-reflect,
+
+  # args
+
+  #   attrset for fetching source code.
+  #    { type, owner, repo, ref }
+  #
+  #   e.g. type="github", owner="rconybea", repo="cmake-examples", ref="ex1b"
+  #
+  #   see [[../flake.nix]]
+  #
+  #cmake-examples-ex1-path
+
+  # someconfigurationoption ? false
+} :
+
+stdenv.mkDerivation (finalattrs:
+  {
+    name = "xo-unit";
+
+    src = (fetchGit {
+      url = "https://github.com/rconybea/xo-unit";
+      version = "1.0";
+      #ref = "ex1";
+      #rev = "c0472c9d7e4d2c53bfb977d3182380832fe96645";
+    });
+
+    cmakeFlags = ["-DCMAKE_MODULE_PATH=${xo-cmake}/share/cmake"];
+    doCheck = true;
+    nativeBuildInputs = [ cmake catch2 ];
+    propagatedBuildInputs = [ ];
+  })