diff --git a/pkgs/xo-ratio.nix b/pkgs/xo-ratio.nix
new file mode 100644
index 00000000..9f25c5cc
--- /dev/null
+++ b/pkgs/xo-ratio.nix
@@ -0,0 +1,79 @@
+{
+  # 1. nixpkgs dependencies
+  # 1.1. python
+  python, pythonPackages,
+
+  # 1.2. particular python packages
+  sphinx ? pythonPackages.sphinx,
+  sphinx-rtd-theme ? pythonPackages.sphinx-rtd-theme,
+  breathe ? pythonPackages.breathe,
+
+  # 1.3. document-generation packages
+  #      use of makeFontsConf adapted from nixpkgs/development/libraries/gtkmm/4.x.nix
+  #
+  doxygen, graphviz,
+  #fontconfig,
+  makeFontsConf,
+
+  # used for graphviz (dot) see docs/Doxyfile.in
+  #helvetica-neue-lt-std,
+
+  # 1.4. c++ build/utest chain
+  stdenv, cmake, catch2, # ... other deps here
+
+  # 2. 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 = ["-DXO_CMAKE_CONFIG_EXECUTABLE=${xo-cmake}/bin/xo-cmake-config"];
+
+    # NOTE: helvetic-neue-lt-std has unfree license
+    #FONTCONFIG_FILE = makeFontsConf { fontDirectories = [ helvetica-neue-lt-std ]; };
+
+    # move HOME so fontconfig can do sensible things
+    buildPhase = ''
+      set -x
+
+      echo "FONTCONFIG_FILE=$FONTCONFIG_FILE"
+
+      #export FONTCONFIG_FILE=$fontconfig.out/etc/fonts/fonts.conf
+      export HOME=$TMPDIR
+      export XDG_CONFIG_HOME=$TMPDIR
+
+      mkdir $XDG_CONFIG_HOME/fontconfig
+
+      #grep xdg $FONTCONFIG_FILE
+
+      #$fontconfig.out/bin/fc-cache -v
+
+      make && make docs
+    '';
+
+    doCheck = true;
+    nativeBuildInputs = [ cmake catch2 doxygen graphviz sphinx sphinx-rtd-theme breathe ];
+    propagatedBuildInputs = [ xo-reflect ];
+  })