* Builtin function `add' to add integers.

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

tests/lang/eval-okay-arithmetic.nix

@@ -0,0 +1,18 @@
+with import ./lib.nix;
+
+let {
+
+  range = first: last: [first] ++ (if first == last then [] else range (builtins.add first 1) last);
+
+  /* Supposedly tail recursive version:
+  
+  range_ = accum: first: last: 
+    if first == last then ([first] ++ accum)
+    else range_ ([first] ++ accum) (builtins.add first 1) last;
+
+  range = range_ [];
+  */
+
+  body = sum (range 1 50);
+
+}