anchor.diff: -{ lib, stdenv, fetchFromGitLab, cmake }:
  +{ lib
  +, stdenv
  +, fetchFromGitLab
  +, cmake
  +}:
   
   stdenv.mkDerivation rec {
     pname = "eigen";
  
positive.code: 
    src = fetchFromGitLab {
      owner = "libeigen";
      repo = "eigen";
      rev = version;
      sha256 = "1i3cvg8d70dk99fl3lrv3wqhfpdnm5kx01fl7r2bz46sk9bphwm1";
    };
  
    patches = [
  
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;
    };
  }
  