anchor.diff:        rev = "V8.8+beta1";
         sha256 = "1ymxyrvjygscxkfj3qkq66skl3vdjhb670rzvsvgmwrjkrakjnfg";
       };
  -  }."${coq.coq-version}"
  +  };
  +  param = params."${coq.coq-version}"
   ; in
   
   stdenv.mkDerivation rec {
  
positive.code: { stdenv, fetchFromGitHub, coq }:
  
  let param =
    {
      "8.6" = {
        rev = "v8.6.0";
  
negative.code:   };
  
    passthru = {
      compatibleCoqVersions = v: builtins.hasAttr v params;
    };
  }
  