anchor.diff:    fetchFromSourceforge =
       { repo
       , rev
  +    , multihash ? ""
       , sha256
       , name ? "${repo}-${rev}"
       }:
       pkgs.fetchzip {
  -      inherit name sha256;
  +      inherit name multihash sha256;
         url = "http://sourceforge.net/code-snapshots/git/"
           + "${lib.substring 0 1 repo}/"
           + "${lib.substring 0 2 repo}/"
  
positive.code:     pkgs.fetchzip {
        inherit
          name
          sha256
          version;
        url = "https://github.com/${owner}/${repo}/archive/${rev}.tar.gz";
  
negative.code:     { owner
      , repo
      , rev
      , multihash ? ""
      , sha256
      , version ? null
      , name ? "${repo}-${rev}"
  