anchor.diff: -{ stdenv, fetchurl, coq, ncurses, which
  +{ stdenv, fetchFromGitHub, coq, ncurses, which
   , graphviz, withDoc ? false
   }:
   
  
positive.code: 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";
      inherit (param) sha256;
    };
  
  
negative.code:   if stdenv.lib.versionAtLeast coq.coq-version "8.6" then
    {
      version = "1.7.0";
      sha256 = "0wnhj9nqpx2bw6n1l4i8jgrw3pjajvckvj3lr4vzjb3my2lbxdd1";
    }
    else if stdenv.lib.versionAtLeast coq.coq-version "8.5" then
    {
      version = "1.6.1";
      sha256 = "1ilw6vm4dlsdv9cd7kmf0vfrh2kkzr45wrqr8m37miy0byzr4p9i";
    }
    else throw "No version of math-comp is available for Coq ${coq.coq-version}";
  
  