anchor.diff:  stdenv.mkDerivation {
     name = "coq${coq.coq-version}-mathcomp-${param.version}";
   
  -  src = fetchurl {
  -    url = "https://github.com/math-comp/math-comp/archive/mathcomp-${param.version}.tar.gz";
  +  src = fetchFromGitHub {
  +    owner = "math-comp";
  +    repo = "math-comp";
  +    rev = "mathcomp-${param.version}";
       inherit (param) sha256;
     };
   
  
positive.code:   if stdenv.lib.versionAtLeast coq.coq-version "8.6" then
    {
      version = "1.7.0";
      sha256 = "05zgyi4wmasi1rcyn5jq42w0bi9713q9m8dl1fdgl66nmacixh39";
    }
    else if stdenv.lib.versionAtLeast coq.coq-version "8.5" then
    {
      version = "1.6.1";
      sha256 = "1j9ylggjzrxz1i2hdl2yhsvmvy5z6l4rprwx7604401080p5sgjw";
    }
    else throw "No version of math-comp is available for Coq ${coq.coq-version}";
  
  
negative.code: { stdenv, fetchFromGitHub, coq, ncurses, which
  , graphviz, withDoc ? false
  }:
  
  