{ pkgs, from-elisp }: {
  org-tangle = block-predicate: text:
    let blocks = (pkgs.callPackage from-elisp { inherit pkgs; }).parseOrgModeBabel text;
        block-to-str = (block:
          if block-predicate { inherit (block) language flags; } then
            block.body
          else
            ""
        );
    in builtins.concatStringsSep "\n" (map block-to-str blocks); 
}