* Added a builtin function `isList' to test whether a value is a list.

With this primitive, a list-flattening function can be implemented
  (NIX-55, example is in tests/lang/eval-okay-flatten.nix).

tests/lang/eval-okay-flatten.exp

@@ -0,0 +1 @@
+Str("1234567")

tests/lang/eval-okay-flatten.nix

@@ -0,0 +1,19 @@
+let {
+
+  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];
+
+  l = ["1" "2" ["3" ["4"] ["5" "6"]] "7"];
+
+  body = concat (flatten l);
+}