anchor.diff:    "--url=${builtins.toString url}"
   ] ++ lib.optionals (branch != null) [
     "--branch=${branch}"
  +] ++ lib.optionals stableVersion [
  +  "--use-stable-version"
  +  "--tag-prefix=${tagPrefix}"
   ]
  -
  
positive.code:     pushd "$tmpdir"
      commit_date="$(${git}/bin/git show -s --pretty='format:%cs')"
      commit_sha="$(${git}/bin/git show -s --pretty='format:%H')"
      popd
      ${coreutils}/bin/rm -rf "$tmpdir"
  
      # update the nix expression
      ${common-updater-scripts}/bin/update-source-version \
          "$UPDATE_NIX_ATTR_PATH" \
          "unstable-$commit_date" \
          --rev="$commit_sha"
    '';
  
  
negative.code: # commit.
  { url ? null # The git url, if empty it will be set to src.url
  , branch ? null
  , stableVersion ? false # Use version format according to RFC 107 (i.e. LAST_TAG+date=YYYY-MM-DD)
  , tagPrefix ? "" # strip this prefix from a tag name when using stable version
  }:
  
  let
  