anchor.diff:        { stdenv, lib, fetchurl, makeWrapper
         , glibc, zlib, readline, openssl, icu, systemd, libossp_uuid
         , pkgconfig, libxml2, tzdata
  +
  +      # This is important to obtain a version of `libpq` that does not depend on systemd.
  +      , enableSystemd ? (lib.versionAtLeast "9.6" version && !stdenv.isDarwin)
   
         # for postgreql.pkgs
         , this, self, newScope, buildEnv
  
positive.code:       "--sysconfdir=/etc"
        "--libdir=$(lib)/lib"
        "--with-system-tzdata=${tzdata}/share/zoneinfo"
        (lib.optionalString (atLeast "9.6" && !stdenv.isDarwin) "--with-systemd")
        (if stdenv.isDarwin then "--with-uuid=e2fs" else "--with-ossp-uuid")
      ] ++ lib.optionals icuEnabled [ "--with-icu" ];
  
  
negative.code:     buildInputs =
        [ zlib readline openssl libxml2 makeWrapper ]
        ++ lib.optionals icuEnabled [ icu ]
        ++ lib.optionals enableSystemd [ systemd ]
        ++ lib.optionals (!stdenv.isDarwin) [ libossp_uuid ];
  
      nativeBuildInputs = lib.optionals icuEnabled [ pkgconfig ];
  