anchor.diff:  
     src = fetchFromGitLab {
       owner = "libeigen";
  -    repo = "eigen";
  +    repo = pname;
       rev = version;
  -    sha256 = "1i3cvg8d70dk99fl3lrv3wqhfpdnm5kx01fl7r2bz46sk9bphwm1";
  +    hash = "sha256-oXJ4V5rakL9EPtQF0Geptl0HMR8700FdSrOB09DbbMQ=";
     };
   
     patches = [
  
positive.code: { lib, stdenv, fetchFromGitLab, cmake }:
  
  stdenv.mkDerivation rec {
    pname = "eigen";
  
negative.code:   nativeBuildInputs = [ cmake ];
  
    meta = with lib; {
      homepage = "https://eigen.tuxfamily.org";
      description = "C++ template library for linear algebra: vectors, matrices, and related algorithms";
      license = licenses.lgpl3Plus;
      maintainers = with maintainers; [ sander raskin ];
      platforms = platforms.unix;
    };
  }
  