diff --git a/examples/rain-world-program.rkt b/examples/rain-world-program.rkt
index 428ad2a..4764753 100644
--- a/examples/rain-world-program.rkt
+++ b/examples/rain-world-program.rkt
@@ -41,10 +41,23 @@
 (define-struct world (sky ;; listof drop 
                           ))
 
+
+
+(define (my-filter f l)
+  (cond
+   [(null? l)
+    '()]
+   [(f (car l))
+    (cons (car l)
+          (my-filter f (cdr l)))]
+   [else
+    (my-filter f (cdr l))]))
+
+
 ;; tick: world -> world
 (define (tick w)
   (make-world 
-   (filter not-on-floor?
+   (my-filter not-on-floor?
            (map drop-descend (cons (random-drop) (world-sky w))))))
 
 
@@ -90,9 +103,20 @@
                a-scene))
 
 
+
+(define (my-foldl f acc lst)
+  (cond
+   [(null? lst)
+    acc]
+   [else
+    (my-foldl f
+              (f (car (car lst)) acc)
+              (cdr lst))]))
+                   
+
 ;; draw: world -> scene
 (define (draw w)
-  (foldl place-drop BACKGROUND (world-sky w)))
+  (my-foldl place-drop BACKGROUND (world-sky w)))
 
 
 
diff --git a/js-assembler/runtime-src/baselib-check.js b/js-assembler/runtime-src/baselib-check.js
index a08d0cb..b289259 100644
--- a/js-assembler/runtime-src/baselib-check.js
+++ b/js-assembler/runtime-src/baselib-check.js
@@ -24,6 +24,29 @@
 	}
     };
 
+    var makeCheckParameterizedArgumentType = function(parameterizedPredicate, 
+                                                      parameterizedPredicateName) {
+	return function(MACHINE, callerName, position) {
+            var args = [];
+            for (var i = 3; i < arguments.length; i++) {
+                args.push(arguments[i]);
+            }
+	    testArgument(
+		MACHINE,
+		parameterizedPredicateName.apply(null, args),
+		function(x) {
+                    return parameterizedPredicate.apply(null, [x].concat(args));
+                },
+		MACHINE.env[MACHINE.env.length - 1 - position],
+		position,
+		callerName);
+	    return MACHINE.env[MACHINE.env.length - 1 - position];
+	}
+    };
+
+
+
+
 
     var makeCheckListofArgumentType = function(predicate, predicateName) {
         var listPredicate = function(x) {
@@ -127,6 +150,14 @@
         plt.baselib.numbers.isNatural,
         'natural');
 
+    var checkNaturalInRange = makeCheckParameterizedArgumentType(
+        function(x, a, b) {
+            return plt.baselib.numbers.isNatural(x) &&
+        }
+        function(a, b) {
+            return plt.baselib.format('natural between ~a and ~a', [a, b])
+        });
+
     var checkInteger = makeCheckArgumentType(
         plt.baselib.numbers.isInteger,
         'integer');
@@ -182,6 +213,7 @@
     exports.testArgument = testArgument;
     exports.testArity = testArity;
     exports.makeCheckArgumentType = makeCheckArgumentType;
+    exports.makeCheckParameterizedArgumentType = makeCheckParameterizedArgumentType;
     exports.makeCheckListofArgumentType = makeCheckListofArgumentType;
 
     exports.checkOutputPort = checkOutputPort;
@@ -192,6 +224,7 @@
     exports.checkReal = checkReal;
     exports.checkNonNegativeReal = checkNonNegativeReal;
     exports.checkNatural = checkNatural;
+    exports.checkNaturalInRange = checkNaturalInRange;
     exports.checkInteger = checkInteger;
     exports.checkRational = checkRational;
     exports.checkPair = checkPair;
diff --git a/js-assembler/runtime-src/baselib-lists.js b/js-assembler/runtime-src/baselib-lists.js
index 7247b77..aa85f9f 100644
--- a/js-assembler/runtime-src/baselib-lists.js
+++ b/js-assembler/runtime-src/baselib-lists.js
@@ -170,7 +170,7 @@
     // Returns true if x is a list (a chain of pairs terminated by EMPTY).
     var isList = function(x) { 
 	while (x !== Empty.EMPTY) {
-	    if (x instanceof Cons){
+	    if (x instanceof Cons) {
 		x = x.rest;
 	    } else {
 		return false;
@@ -191,6 +191,17 @@
     };
 
 
+    var length = function(lst) {
+        var len = 0;
+        while (lst !== EMPTY) {
+            len++;
+            lst = lst.rest;
+        }
+        return len;
+    };
+
+
+
     //////////////////////////////////////////////////////////////////////
 
     exports.EMPTY = EMPTY;
@@ -202,6 +213,7 @@
     exports.makePair = makePair;
     exports.makeList = makeList;
     exports.reverse = reverse;
+    exports.length = length;
 
 
 })(this['plt'].baselib);
\ No newline at end of file
diff --git a/js-assembler/runtime-src/runtime.js b/js-assembler/runtime-src/runtime.js
index ee4767a..8b5fb10 100644
--- a/js-assembler/runtime-src/runtime.js
+++ b/js-assembler/runtime-src/runtime.js
@@ -115,6 +115,7 @@ if(this['plt'] === undefined) { this['plt'] = {}; }
     var checkReal = plt.baselib.check.checkReal;
     var checkNonNegativeReal = plt.baselib.check.checkNonNegativeReal;
     var checkNatural = plt.baselib.check.checkNatural;
+    var checkNaturalInRange = plt.baselib.check.checkNaturalInRange;
     var checkInteger = plt.baselib.check.checkInteger;
     var checkRational = plt.baselib.check.checkRational;
     var checkPair = plt.baselib.check.checkPair;
@@ -828,6 +829,19 @@ if(this['plt'] === undefined) { this['plt'] = {}; }
 	    return result;
         });
 
+    installPrimitiveProcedure(
+        'list-ref',
+        2,
+        function(MACHINE) {
+            var lst = checkList(MACHINE, 'list-ref', 0);
+            var index = checkNaturalInRange(MACHINE, 'list-ref', 1,
+                                            0, plt.baselib.lists.length(lst));
+
+        });
+
+
+
+
     installPrimitiveProcedure(
         'car',
         1,
@@ -951,7 +965,8 @@ if(this['plt'] === undefined) { this['plt'] = {}; }
 	    var elts = checkVector(MACHINE, 'vector-set!', 0).elts;
             // FIXME: check out-of-bounds vector
 	    var index = plt.baselib.numbers.toFixnum(
-                checkNatural(MACHINE, 'vector-set!', 1));
+                checkNaturalInRange(MACHINE, 'vector-set!', 1,
+                                    0, elts.length));
 	    var val = MACHINE.env[MACHINE.env.length - 1 - 2];
 	    elts[index] = val;
 	    return VOID;
diff --git a/lang/kernel.rkt b/lang/kernel.rkt
index e3f778a..ca26de0 100644
--- a/lang/kernel.rkt
+++ b/lang/kernel.rkt
@@ -291,7 +291,7 @@ exact?
   length
 ;;  list?
 ;;  list*
-;;  list-ref
+  list-ref
 ;;  list-tail
   append
   reverse