* Builtin function `add' to add integers.

* Put common test functions in tests/lang/lib.nix.

tests/lang/lib.nix

@@ -0,0 +1,18 @@
+rec {
+
+  fold = op: nul: list:
+    if list == []
+    then nul
+    else op (builtins.head list) (fold op nul (builtins.tail list));
+
+  concat =
+    fold (x: y: x + y) "";
+
+  flatten = x:
+    if builtins.isList x
+    then fold (x: y: (flatten x) ++ y) [] x
+    else [x];
+
+  sum = fold (x: y: builtins.add x y) 0;
+
+}