Add '33.nix'

33.nix

@@ -0,0 +1,11 @@
+with import <nixpkgs> { };
+let
+  list = [["a"] ["b"] ["c"]];
+  
+  attrList = [{a = 1;} {b = 0;} {a = 2;}];
+  catAttrs = atr: l: lib.fold (val: _l: if (builtins.hasAttr atr val) then [ (val.${atr}) ] ++ _l else _l) [] l;
+in
+rec {
+  example = builtins.concatLists list; #is [ "a" "b" "c" ]
+  result = catAttrs "a" attrList; #should be [1 2] 
+}