anchor.diff:  { stdenv
   , autoreconfHook
  -, fetchgit
  +, fetchFromGitHub
   , fetchurl
   , lib
   
  
positive.code:     };
      "head" = {
        fetchzipversion = 3;
        version = "2017-09-01";
        rev = "95c48711f60092ad9108fe78d4b84c5a3eac84ad";
        sha256 = "2f5e993dfef17ee621804730e4f1db14dc75ac03f69ee6483b3fff709c81230d";
      };
    };
    source = sources."${channel}";
  
negative.code: 
    src =
      if channel == "head" then
        fetchFromGitHub {
          version = source.fetchzipversion;
          owner = "xiph";
          repo = "opus";
          inherit (source) rev sha256;
        }
      else
  