anchor.diff:  , zlib
   }:
   
  +# NOTE: as of late 2020, yosys has switched to an automation robot that
  +# automatically tags their repository Makefile with a new build number every
  +# day when changes are committed. please MAKE SURE that the version number in
  +# the 'version' field exactly matches the YOSYS_VER field in the Yosys
  +# makefile!
  +#
  +# if a change in yosys isn't yet available under a build number like this (i.e.
  +# it was very recently merged, within an hour), wait a few hours for the
  +# automation robot to tag the new version, like so:
  +#
  +#     https://github.com/YosysHQ/yosys/commit/71ca9a825309635511b64b3ec40e5e5e9b6ad49b
  +#
  +# note that while most nix packages for "unstable versions" use a date-based
  +# version scheme, synchronizing the nix package version here with the unstable
  +# yosys version number helps users report better bugs upstream, and is
  +# ultimately less confusing than using dates.
  +
   stdenv.mkDerivation rec {
     pname   = "yosys";
  -  version = "2020.10.20";
  +  version = "0.9+3675";
   
     src = fetchFromGitHub {
       owner  = "YosysHQ";
       repo   = "yosys";
  -    rev    = "06347b119b08257eff37cdd10ed802e794c1a3cf";
  -    sha256 = "07058r8095192gwkiy2yyvj9vxv09fcw9bqi2b39wn85lh5kx2k2";
  +    rev    = "71ca9a825309635511b64b3ec40e5e5e9b6ad49b";
  +    sha256 = "03jlhfvm5rxx8yybf94nqd3ld2y6brp8r0k6gfi56chv3iqqavy3";
     };
   
     enableParallelBuilding = true;
  
positive.code:   # they just assume that 'yosys-abc' is available -- but it's not installed
    # when using ABCEXTERNAL
    #
    # add a symlink to fake things so that both variants work the same way.
    postInstall = ''
      ln -sfv ${abc-verifier}/bin/abc $out/bin/yosys-abc
    '';
  
    meta = with stdenv.lib; {
      description = "Open RTL synthesis framework and tools";
  
negative.code:     (cd misc && ${protobuf}/bin/protoc --cpp_out ../backends/protobuf/ ./yosys.proto)
  
      if ! grep -q "ABCREV = ${shortAbcRev}" Makefile; then
        echo "ERROR: yosys isn't compatible with the provided abc (${shortAbcRev}), failing."
        exit 1
      fi
  
      if ! grep -q "YOSYS_VER := ${version}" Makefile; then
        echo "ERROR: yosys version in Makefile isn't equivalent to version of the nix package (${version}), failing."
        exit 1
      fi
    '';
  
    checkTarget = "test";
    doCheck = true;
    checkInputs = [ verilog ];
  
  