Add '21.nix'

21.nix

@@ -0,0 +1,14 @@
+let
+  attrSet = {x = "a"; y = "b"; b = {t = true; f = false;};};
+  attrSet.c = 1;
+  attrSet.d = 2;
+  attrSet.e.f = "g";
+
+  list1 = [attrSet.c attrSet.d];
+  list2 = [attrSet.x attrSet.y];
+
+in
+{
+  #List concatenation.
+  ex0 = ["a" "b" 1 2] == [ attrSet.x attrSet.y ] ++ [attrSet.c attrSet.d ];
+}