diff --git a/cli/TODO.txt b/cli/TODO.txt
index fea3a00..617107a 100644
--- a/cli/TODO.txt
+++ b/cli/TODO.txt
@@ -1,25 +1,30 @@
 
 // To implement.
 
-[ ] Implement argparse.
-[ ] Implement resolve path in cli.
+- Implement argparse.
+    - -v --version
+    - emit opcodes
+    - maybe write a similer .pyc file for pocket
 
-[ ] Structs.
-[ ] Implement math library.
-[ ] Single header for embedding.
-[ ] Implement fiber from script body and vm run fibers (not scripts).
+- Ignore line with '\' character.
+- Implement resolve path in cli.
+
+- Structs.
+- Implement math library.
+- Single header for embedding.
+- Implement fiber from script body and vm run fibers (not scripts).
     Then remove vm's root script.
-[ ] C Integration api (including add core lib from host application).
-[ ] REPL.
-    [ ] compile expression for a script.
-[ ] Var handler implement.
-[ ] Make it possible to override function names.
+- C Integration api (including add core lib from host application).
+- REPL.
+    - compile expression for a script.
+- Var handler implement.
+- Make it possible to override function names.
       - To do so the functions and global variables should be in the same
         buffer as the property of the script.
-[ ] Hex, binary literals and floats like ".5".
-[ ] Function docstring property.
-[ ] Union tagging alter in var.
-[ ] (future) add structs and maybe enums.
+- Hex, binary literals and floats like ".5".
+- Function docstring property.
+- Union tagging alter in var.
+- (future) add structs and maybe enums.
 
 // Add more.
 [ ] Compilte core methods.
diff --git a/cli/cli_modules.c b/cli/cli_modules.c
index 5aee559..feb9651 100644
--- a/cli/cli_modules.c
+++ b/cli/cli_modules.c
@@ -36,8 +36,10 @@ void register_cli_modules(PKVM* vm) {
   PkHandle* path = pkNewModule(vm, "path");
   pkModuleAddFunction(vm, path, "abspath", stdPathAbspath, 1);
   pkModuleAddFunction(vm, path, "curdir", stdPathCurdir, 0);
+  pkReleaseHandle(vm, path);
 
   PkHandle* test = pkNewModule(vm, "test");
   pkModuleAddFunction(vm, test, "add", testAdd, 2);
+  pkReleaseHandle(vm, test);
 
 }
\ No newline at end of file
diff --git a/cli/main.c b/cli/main.c
index 0fe44c4..84ed727 100644
--- a/cli/main.c
+++ b/cli/main.c
@@ -105,7 +105,6 @@ int main(int argc, char** argv) {
   config.load_script_fn = loadScript;
   config.resolve_path_fn = resolvePath;
 
-
   // FIXME: this is temp till arg parse implemented.
   if (argc >= 3 && strcmp(argv[1], "-c") == 0) {
     PKVM* vm = pkNewVM(&config);
diff --git a/docs/try/main.c b/docs/try/main.c
index 30080c7..6dfe0bb 100644
--- a/docs/try/main.c
+++ b/docs/try/main.c
@@ -14,7 +14,7 @@ extern void js_errorPrint(int type, int line, const char* message);
 extern void js_writeFunction(const char* message);
 extern const char* js_loadScript();
 
-void errorPrint(PKVM* vm, PKErrorType type, const char* file, int line,
+void errorPrint(PKVM* vm, PkErrorType type, const char* file, int line,
                 const char* message) {
   // No need to pass file (since there is only script that'll ever run on the 
   // browser.
@@ -47,7 +47,7 @@ int runSource(const char* source) {
   config.resolve_path_fn = resolvePath;
 
   PKVM* vm = pkNewVM(&config);
-  PKInterpretResult result = pkInterpretSource(vm, source, "@try");
+  PkInterpretResult result = pkInterpretSource(vm, source, "@try");
 
   pkFreeVM(vm);
 
diff --git a/src/core.c b/src/core.c
index 6c5a1c8..0c2a1c5 100644
--- a/src/core.c
+++ b/src/core.c
@@ -59,7 +59,7 @@ void pkModuleAddFunction(PKVM* vm, PkHandle* module, const char* name,
 // Check for errors in before calling the get arg public api function.
 #define CHECK_GET_ARG_API_ERRORS()                                             \
   __ASSERT(vm->fiber != NULL, "This function can only be called at runtime."); \
-  __ASSERT(arg > 0 || arg < ARGC, "Invalid argument index.");                  \
+  __ASSERT(arg > 0 || arg <= ARGC, "Invalid argument index.");                  \
   __ASSERT(value != NULL, "Parameter [value] was NULL.");                      \
   ((void*)0)
 
@@ -79,7 +79,7 @@ int pkGetArgc(PKVM* vm) {
 
 PkVar pkGetArg(PKVM* vm, int arg) {
   __ASSERT(vm->fiber != NULL, "This function can only be called at runtime.");
-  __ASSERT(arg > 0 || arg < ARGC, "Invalid argument index.");
+  __ASSERT(arg > 0 || arg <= ARGC, "Invalid argument index.");
 
   return &(ARG(arg));
 }
@@ -217,19 +217,23 @@ static inline bool validateIndex(PKVM* vm, int32_t index, int32_t size,
   return true;
 }
 
-// Check if [var] is string for argument at [arg_ind]. If not set error and
+// Check if [var] is string for argument at [arg]. If not set error and
 // return false.
-static bool validateArgString(PKVM* vm, Var var, String** value, int arg_ind) {
-  if (!IS_OBJ(var) || AS_OBJ(var)->type != OBJ_STRING) {
-    String* str_arg = toString(vm, VAR_NUM((double)arg_ind), false);
-    vmPushTempRef(vm, &str_arg->_super);
-    vm->fiber->error = stringFormat(vm, "Expected a string at argument @.",
-                                    str_arg, false);
-    vmPopTempRef(vm);
-  }
-  *value = (String*)AS_OBJ(var);
-  return true;
-}
+#define VALIDATE_ARG_OBJ(m_class, m_type, m_name)                            \
+  static bool validateArg##m_class(PKVM* vm, int arg, m_class** value) {     \
+    Var var = ARG(arg);                                                      \
+    ASSERT(arg > 0 && arg <= ARGC, OOPS);                                    \
+    if (!IS_OBJ(var) || AS_OBJ(var)->type != m_type) {                       \
+      char buff[12]; sprintf(buff, "%d", arg);                               \
+      vm->fiber->error = stringFormat(vm, "Expected a " m_name               \
+        " at argument $.", buff, false);                                     \
+    }                                                                        \
+    *value = (m_class*)AS_OBJ(var);                                          \
+    return true;                                                             \
+   }
+ VALIDATE_ARG_OBJ(String, OBJ_STRING, "string")
+ VALIDATE_ARG_OBJ(List, OBJ_LIST, "list")
+ VALIDATE_ARG_OBJ(Map, OBJ_MAP, "map")
 
 /*****************************************************************************/
 /* BUILTIN FUNCTIONS API                                                     */
@@ -284,6 +288,10 @@ FN_IS_OBJ_TYPE(Function,  OBJ_FUNC)
 FN_IS_OBJ_TYPE(Script,  OBJ_SCRIPT)
 FN_IS_OBJ_TYPE(UserObj,  OBJ_USER)
 
+void coreTypeName(PKVM* vm) {
+  RET(VAR_OBJ(&newString(vm, varTypeName(ARG1))->_super));
+}
+
 void coreAssert(PKVM* vm) {
   int argc = ARGC;
   if (argc != 1 && argc != 2) {
@@ -346,12 +354,12 @@ void corePrint(PKVM* vm) {
   vm->config.write_fn(vm, "\n");
 }
 
-// string functions
-// ----------------
+// String functions.
+// -----------------
 
 void coreStrLower(PKVM* vm) {
   String* str;
-  if (!validateArgString(vm, ARG1, &str, 1)) return;
+  if (!validateArgString(vm, 1, &str)) return;
 
   String* result = newStringLength(vm, str->data, str->length);
   char* data = result->data;
@@ -364,7 +372,7 @@ void coreStrLower(PKVM* vm) {
 
 void coreStrUpper(PKVM* vm) {
   String* str;
-  if (!validateArgString(vm, ARG1, &str, 1)) return;
+  if (!validateArgString(vm, 1, &str)) return;
 
   String* result = newStringLength(vm, str->data, str->length);
   char* data = result->data;
@@ -377,7 +385,7 @@ void coreStrUpper(PKVM* vm) {
 
 void coreStrStrip(PKVM* vm) {
   String* str;
-  if (!validateArgString(vm, ARG1, &str, 1)) return;
+  if (!validateArgString(vm, 1, &str)) return;
 
   const char* start = str->data;
   while (*start && isspace(*start)) start++;
@@ -389,6 +397,29 @@ void coreStrStrip(PKVM* vm) {
   RET(VAR_OBJ(&newStringLength(vm, start, (uint32_t)(end - start + 1))->_super));
 }
 
+// List functions.
+// ---------------
+void coreListAppend(PKVM* vm) {
+  List* list;
+  if (!validateArgList(vm, 1, &list)) return;
+  Var elem = ARG(2);
+
+  varBufferWrite(&list->elements, vm, elem);
+  RET(VAR_OBJ(&list->_super));
+}
+
+// Map functions.
+// --------------
+
+void coreMapRemove(PKVM* vm) {
+  Map* map;
+  if (!validateArgMap(vm, 1, &map)) return;
+  Var key = ARG(2);
+
+  mapRemoveKey(vm, map, key);
+  RET(VAR_OBJ(&map->_super));
+}
+
 /*****************************************************************************/
 /* CORE MODULE METHODS                                                       */
 /*****************************************************************************/
@@ -456,6 +487,11 @@ void stdLangGC(PKVM* vm) {
   RET(VAR_NUM((double)garbage));
 }
 
+// A debug function for development (will be removed).
+void stdLangDebugBreak(PKVM* vm) {
+  DEBUG_BREAK();
+}
+
 // Write function, just like print function but it wont put space between args
 // and write a new line at the end.
 void stdLangWrite(PKVM* vm) {
@@ -488,6 +524,8 @@ void initializeCore(PKVM* vm) {
                       (int)strlen(name), argc, fn);
 
   // Initialize builtin functions.
+  INITALIZE_BUILTIN_FN("type_name",   coreTypeName,   1);
+
   INITALIZE_BUILTIN_FN("is_null",     coreIsNull,     1);
   INITALIZE_BUILTIN_FN("is_bool",     coreIsBool,     1);
   INITALIZE_BUILTIN_FN("is_num",      coreIsNum,      1);
@@ -510,12 +548,19 @@ void initializeCore(PKVM* vm) {
   INITALIZE_BUILTIN_FN("str_upper",   coreStrUpper,   1);
   INITALIZE_BUILTIN_FN("str_strip",   coreStrStrip,   1);
 
+  // List functions.
+  INITALIZE_BUILTIN_FN("list_append", coreListAppend, 2);
+
+  // Map functions.
+  INITALIZE_BUILTIN_FN("map_remove",  coreMapRemove,  2);
+
   // Core Modules /////////////////////////////////////////////////////////////
 
   Script* lang = newModuleInternal(vm, "lang");
   moduleAddFunctionInternal(vm, lang, "clock", stdLangClock,  0);
   moduleAddFunctionInternal(vm, lang, "gc",    stdLangGC,     0);
   moduleAddFunctionInternal(vm, lang, "write", stdLangWrite, -1);
+  moduleAddFunctionInternal(vm, lang, "debug_break", stdLangDebugBreak, 0);
 }
 
 /*****************************************************************************/
@@ -543,7 +588,7 @@ Var varAdd(PKVM* vm, Var v1, Var v2) {
       case OBJ_STRING:
       {
         if (o2->type == OBJ_STRING) {
-          return VAR_OBJ(stringFormat(vm, "@@", v1, v2));
+          return VAR_OBJ(stringFormat(vm, "@@", (String*)o1, (String*)o2));
         }
       } break;
 
@@ -623,7 +668,7 @@ Var varModulo(PKVM* vm, Var v1, Var v2) {
   return VAR_NULL;
 }
 
-bool varGreater(PKVM* vm, Var v1, Var v2) {
+bool varGreater(Var v1, Var v2) {
   double d1, d2;
   if (isNumeric(v1, &d1) && isNumeric(v2, &d2)) {
     return d1 > d2;
@@ -633,7 +678,7 @@ bool varGreater(PKVM* vm, Var v1, Var v2) {
   return false;
 }
 
-bool varLesser(PKVM* vm, Var v1, Var v2) {
+bool varLesser(Var v1, Var v2) {
   double d1, d2;
   if (isNumeric(v1, &d1) && isNumeric(v2, &d2)) {
     return d1 < d2;
diff --git a/src/core.h b/src/core.h
index 1b42721..312a394 100644
--- a/src/core.h
+++ b/src/core.h
@@ -34,8 +34,8 @@ Var varMultiply(PKVM* vm, Var v1, Var v2);
 Var varDivide(PKVM* vm, Var v1, Var v2);
 Var varModulo(PKVM* vm, Var v1, Var v2);
 
-bool varGreater(PKVM* vm, Var v1, Var v2);
-bool varLesser(PKVM* vm, Var v1, Var v2);
+bool varGreater(Var v1, Var v2);
+bool varLesser(Var v1, Var v2);
 
 Var varGetAttrib(PKVM* vm, Var on, String* attrib);
 void varSetAttrib(PKVM* vm, Var on, String* name, Var value);
diff --git a/src/debug.c b/src/debug.c
index 310074b..7a3dd1d 100644
--- a/src/debug.c
+++ b/src/debug.c
@@ -9,6 +9,9 @@
 #include "debug.h"
 #include "vm.h"
 
+// To limit maximum elements to be dumpin in a map or a list.
+#define MAX_DUMP_ELEMENTS 30
+
 static const char* op_name[] = {
   #define OPCODE(name, params, stack) #name,
   #include "opcodes.h"
@@ -45,6 +48,12 @@ static void _dumpValue(PKVM* vm, Var value, bool recursive) {
         for (uint32_t i = 0; i < list->elements.count; i++) {
           if (i != 0) printf(", ");
           _dumpValue(vm, list->elements.data[i], true);
+
+          // Terminate the dump if it's too long.
+          if (i >= MAX_DUMP_ELEMENTS) {
+            printf("...");
+            break;
+          }
         }
         printf("]");
       }
@@ -58,11 +67,21 @@ static void _dumpValue(PKVM* vm, Var value, bool recursive) {
         printf("{...}");
       } else {
         printf("{");
-        for (uint32_t i = 0; i < map->count; i++) {
-          if (i != 0) printf(", ");
+        bool first = true;
+        for (uint32_t i = 0; i < map->capacity; i++) {
+          if (IS_UNDEF(map->entries[i].key)) continue;
+          if (!first) printf(", "); first = false;
+
           _dumpValue(vm, map->entries[i].key, true);
           printf(":");
           _dumpValue(vm, map->entries[i].value, true);
+
+          // Terminate the dump if it's too long.
+          if (i >= MAX_DUMP_ELEMENTS) {
+            printf("...");
+            break;
+          }
+
         }
         printf("}");
       }
diff --git a/src/var.c b/src/var.c
index 5195d4b..fc5b754 100644
--- a/src/var.c
+++ b/src/var.c
@@ -142,8 +142,8 @@ static void blackenObject(Object* obj, PKVM* vm) {
       Map* map = (Map*)obj;
       for (uint32_t i = 0; i < map->capacity; i++) {
         if (IS_UNDEF(map->entries[i].key)) continue;
-        grayObject(vm, AS_OBJ(map->entries[i].key));
-        grayObject(vm, AS_OBJ(map->entries[i].value));
+        grayValue(vm, map->entries[i].key);
+        grayValue(vm, map->entries[i].value);
       }
       vm->bytes_allocated += sizeof(Map);
       vm->bytes_allocated += sizeof(MapEntry) * map->capacity;
@@ -598,9 +598,17 @@ Var mapRemoveKey(PKVM* vm, Map* self, Var key) {
     // Clear the map if it's empty.
     mapClear(vm, self);
 
-  } else if (self->capacity > MIN_CAPACITY &&
-             self->capacity / GROW_FACTOR > self->count / MAP_LOAD_PERCENT * 100) {
-    uint32_t capacity = self->capacity / GROW_FACTOR;
+ } else if ((self->capacity > MIN_CAPACITY) &&
+             (self->capacity / (GROW_FACTOR * GROW_FACTOR)) >
+             ((self->count * 100) / MAP_LOAD_PERCENT)) {
+
+    // We grow the map when it's filled 75% (MAP_LOAD_PERCENT) by 2
+    // (GROW_FACTOR) but we're not shrink the map when it's half filled (ie.
+    // half of the capacity is 75%). Instead we wait till it'll become 1/4 is
+    // filled (1/4 = 1/(GROW_FACTOR*GROW_FACTOR)) to minimize the
+    // reallocations and which is more faster.
+
+    uint32_t capacity = self->capacity / (GROW_FACTOR * GROW_FACTOR);
     if (capacity < MIN_CAPACITY) capacity = MIN_CAPACITY;
 
     _mapResize(vm, self, capacity);
@@ -778,28 +786,44 @@ String* toString(PKVM* vm, Var v, bool recursive) {
         // If recursive return with quotes (ex: [42, "hello", 0..10]).
         String* string = newStringLength(vm, ((String*)obj)->data, ((String*)obj)->length);
         if (!recursive) return string;
-        else return stringFormat(vm, "\"@\"", string);
+        vmPushTempRef(vm, &string->_super);
+        String* repr = stringFormat(vm, "\"@\"", string);
+        vmPopTempRef(vm);
+        return repr;
       }
 
-      case OBJ_LIST: {
+      case OBJ_LIST:
+      {
         List* list = (List*)obj;
+        if (list->elements.count == 0) return newStringLength(vm, "[]", 2);
+
         String* result = newStringLength(vm, "[", 1);
+        vmPushTempRef(vm, &result->_super); // result
 
         for (uint32_t i = 0; i < list->elements.count; i++) {
           const char* fmt = (i != 0) ? "@, @" : "@@";
 
-          vmPushTempRef(vm, &result->_super);
-          result = stringFormat(vm, fmt, result,
-            toString(vm, list->elements.data[i], true));
-          vmPopTempRef(vm);
+          String* elem_str = toString(vm, list->elements.data[i], true);
+          vmPushTempRef(vm, &elem_str->_super); // elem_str
+          result = stringFormat(vm, fmt, result, elem_str);
+          vmPopTempRef(vm); // elem_str
+
+          vmPopTempRef(vm); // result (old pointer)
+          vmPushTempRef(vm, &result->_super); // result (old pointer)
         }
-        return stringFormat(vm, "@]", result);
+
+        result = stringFormat(vm, "@]", result);
+        vmPopTempRef(vm); // result (last pointer)
+        return result;
       }
 
       case OBJ_MAP:
       {
         Map* map = (Map*)obj;
+        if (map->entries == NULL) return newStringLength(vm, "{}", 2);
+
         String* result = newStringLength(vm, "{", 1);
+        vmPushTempRef(vm, &result->_super); // result
 
         uint32_t i = 0;
         bool _first = true; // For first element no ',' required.
@@ -817,34 +841,40 @@ String* toString(PKVM* vm, Var v, bool recursive) {
           if (_done) break;
 
           const char* fmt = (!_first) ? "@, @:@" : "@@:@";
-          vmPushTempRef(vm, &result->_super);
-          result = stringFormat(vm, fmt, result,
-            toString(vm, map->entries[i].key, true),
-            toString(vm, map->entries[i].value, true));
-          vmPopTempRef(vm);
+
+          String* key_str = toString(vm, map->entries[i].key, true);
+          vmPushTempRef(vm, &key_str->_super); // key_str
+
+          String* val_str = toString(vm, map->entries[i].value, true);
+          vmPushTempRef(vm, &val_str->_super); // val_str
+
+          result = stringFormat(vm, fmt, result, key_str, val_str);
+          vmPopTempRef(vm); // val_str
+          vmPopTempRef(vm); // key_str
+
+          vmPopTempRef(vm); // result (old pointer)
+          vmPushTempRef(vm, &result->_super); // result (new pointer)
 
           _first = false;
           i++;
         } while (i < map->capacity);
 
-        return stringFormat(vm, "@}", result);
+        result = stringFormat(vm, "@}", result);
+        vmPopTempRef(vm); // result (last pointer)
+        return result;
       }
 
       case OBJ_RANGE:
       {
         Range* range = (Range*)obj;
 
-        String* from = toString(vm, VAR_NUM(range->from), false);
-        vmPushTempRef(vm, &from->_super);
+        // FIXME: Validate I might need some review on the below one.
+        char buff_from[50];
+        snprintf(buff_from, 50, "%f", range->from);
+        char buff_to[50];
+        snprintf(buff_to, 50, "%f", range->to);
 
-        String* to = toString(vm, VAR_NUM(range->to), false);
-        vmPushTempRef(vm, &from->_super);
-
-        String* str = stringFormat(vm, "[Range:@..@]", from, to);
-        vmPopTempRef(vm); // to.
-        vmPopTempRef(vm); // from.
-
-        return str;
+        return stringFormat(vm, "[Range:$..$]", buff_from, buff_to);
       }
 
       case OBJ_SCRIPT: {
@@ -904,7 +934,7 @@ String* stringFormat(PKVM* vm, const char* fmt, ...) {
         break;
 
       case '@':
-        total_length += AS_STRING(va_arg(arg_list, Var))->length;
+        total_length += va_arg(arg_list, String*)->length;
         break;
 
       default:
@@ -929,7 +959,7 @@ String* stringFormat(PKVM* vm, const char* fmt, ...) {
 
       case '@':
       {
-        String* string = AS_STRING(va_arg(arg_list, Var));
+        String* string = va_arg(arg_list, String*);
         memcpy(buff, string->data, string->length);
         buff += string->length;
       } break;
diff --git a/src/vm.c b/src/vm.c
index 82d1ef6..e884b01 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -29,7 +29,7 @@
 
 // The heap size for the next GC will be calculated as the bytes we have
 // allocated so far plus the fill factor of it.
-#define HEAP_FILL_PERCENT 50
+#define HEAP_FILL_PERCENT 75
 
 static void* defaultRealloc(void* memory, size_t new_size, void* user_data) {
   if (new_size == 0) {
@@ -109,7 +109,7 @@ void pkFreeVM(PKVM* self) {
 
   // Tell the host application that it forget to release all of it's handles
   // before freeing the VM.
-  __ASSERT(self->handles != NULL, "Not all handles were released.");
+  __ASSERT(self->handles == NULL, "Not all handles were released.");
 
   DEALLOCATE(self, self);
 }
@@ -497,7 +497,7 @@ PkInterpretResult vmRunScript(PKVM* vm, Script* _script) {
 #define PUSH(value)  (*vm->fiber->sp++ = (value))
 #define POP()        (*(--vm->fiber->sp))
 #define DROP()       (--vm->fiber->sp)
-#define PEEK()       (*(vm->fiber->sp - 1))
+#define PEEK(off)    (*(vm->fiber->sp + (off)))
 #define READ_BYTE()  (*IP++)
 #define READ_SHORT() (IP+=2, (uint16_t)((IP[-2] << 8) | IP[-1]))
 
@@ -533,11 +533,11 @@ PkInterpretResult vmRunScript(PKVM* vm, Script* _script) {
 #endif
 
 #if  DEBUG_DUMP_CALL_STACK
-  #define DEBUG_CALL_STACK()      \
-    do {                          \
-      /*   system("cls");   */    \
-      dumpGlobalValues(vm);       \
-      dumpStackFrame(vm);         \
+  #define DEBUG_CALL_STACK()        \
+    do {                            \
+      system("cls");                \
+      dumpGlobalValues(vm);         \
+      dumpStackFrame(vm);           \
     } while (false)
 #else
   #define DEBUG_CALL_STACK() ((void*)0)
@@ -603,18 +603,19 @@ PkInterpretResult vmRunScript(PKVM* vm, Script* _script) {
 
     OPCODE(LIST_APPEND):
     {
-      Var elem = POP();
-      Var list = PEEK();
+      Var elem = PEEK(-1); // Don't pop yet, we need the reference for gc.
+      Var list = PEEK(-2);
       ASSERT(IS_OBJ(list) && AS_OBJ(list)->type == OBJ_LIST, OOPS);
       varBufferWrite(&((List*)AS_OBJ(list))->elements, vm, elem);
+      POP(); // elem
       DISPATCH();
     }
 
     OPCODE(MAP_INSERT):
     {
-      Var value = POP();
-      Var key = POP();
-      Var on = PEEK();
+      Var value = PEEK(-1); // Don't pop yet, we need the reference for gc.
+      Var key = PEEK(-2);   // Don't pop yet, we need the reference for gc. 
+      Var on = PEEK(-3);
 
       ASSERT(IS_OBJ(on) && AS_OBJ(on)->type == OBJ_MAP, OOPS);
 
@@ -623,8 +624,11 @@ PkInterpretResult vmRunScript(PKVM* vm, Script* _script) {
       } else {
         mapSet(vm, (Map*)AS_OBJ(on), key, value);
       }
-
       varsetSubscript(vm, on, key, value);
+
+      POP(); // value
+      POP(); // key
+
       CHECK_ERROR();
       DISPATCH();
     }
@@ -661,13 +665,13 @@ PkInterpretResult vmRunScript(PKVM* vm, Script* _script) {
     OPCODE(STORE_LOCAL_8):
     {
       int index = (int)(instruction - OP_STORE_LOCAL_0);
-      rbp[index + 1] = PEEK();  // +1: rbp[0] is return value.
+      rbp[index + 1] = PEEK(-1);  // +1: rbp[0] is return value.
       DISPATCH();
     }
     OPCODE(STORE_LOCAL_N):
     {
       uint16_t index = READ_SHORT();
-      rbp[index + 1] = PEEK();  // +1: rbp[0] is return value.
+      rbp[index + 1] = PEEK(-1);  // +1: rbp[0] is return value.
       DISPATCH();
     }
 
@@ -683,7 +687,7 @@ PkInterpretResult vmRunScript(PKVM* vm, Script* _script) {
     {
       uint16_t index = READ_SHORT();
       ASSERT(index < script->globals.count, OOPS);
-      script->globals.data[index] = PEEK();
+      script->globals.data[index] = PEEK(-1);
       DISPATCH();
     }
 
@@ -841,16 +845,19 @@ PkInterpretResult vmRunScript(PKVM* vm, Script* _script) {
 
     OPCODE(GET_ATTRIB):
     {
-      Var on = POP();
+      Var on = PEEK(-1); // Don't pop yet, we need the reference for gc. 
       String* name = script->names.data[READ_SHORT()];
-      PUSH(varGetAttrib(vm, on, name));
+      Var value = varGetAttrib(vm, on, name);
+      POP(); // on
+      PUSH(value);
+
       CHECK_ERROR();
       DISPATCH();
     }
 
     OPCODE(GET_ATTRIB_KEEP):
     {
-      Var on = PEEK();
+      Var on = PEEK(-1);
       String* name = script->names.data[READ_SHORT()];
       PUSH(varGetAttrib(vm, on, name));
       CHECK_ERROR();
@@ -859,28 +866,36 @@ PkInterpretResult vmRunScript(PKVM* vm, Script* _script) {
 
     OPCODE(SET_ATTRIB):
     {
-      Var value = POP();
-      Var on = POP();
+      Var value = PEEK(-1); // Don't pop yet, we need the reference for gc. 
+      Var on = PEEK(-2);    // Don't pop yet, we need the reference for gc. 
       String* name = script->names.data[READ_SHORT()];
       varSetAttrib(vm, on, name, value);
+
+      POP(); // value
+      POP(); // on
       PUSH(value);
+
       CHECK_ERROR();
       DISPATCH();
     }
 
     OPCODE(GET_SUBSCRIPT):
     {
-      Var key = POP();
-      Var on = POP();
-      PUSH(varGetSubscript(vm, on, key));
+      Var key = PEEK(-1); // Don't pop yet, we need the reference for gc.
+      Var on = PEEK(-2);  // Don't pop yet, we need the reference for gc.
+      Var value = varGetSubscript(vm, on, key);
+      POP(); // key
+      POP(); // on
+      PUSH(value);
+
       CHECK_ERROR();
       DISPATCH();
     }
 
     OPCODE(GET_SUBSCRIPT_KEEP):
     {
-      Var key = *(vm->fiber->sp - 1);
-      Var on = *(vm->fiber->sp - 2);
+      Var key = PEEK(-1);
+      Var on = PEEK(-2);
       PUSH(varGetSubscript(vm, on, key));
       CHECK_ERROR();
       DISPATCH();
@@ -888,14 +903,16 @@ PkInterpretResult vmRunScript(PKVM* vm, Script* _script) {
 
     OPCODE(SET_SUBSCRIPT):
     {
-      Var value = POP();
-      Var key = POP();
-      Var on = POP();
-
+      Var value = PEEK(-1); // Don't pop yet, we need the reference for gc.
+      Var key = PEEK(-2);   // Don't pop yet, we need the reference for gc.
+      Var on = PEEK(-3);    // Don't pop yet, we need the reference for gc.
       varsetSubscript(vm, on, key, value);
-      CHECK_ERROR();
+      POP(); // value
+      POP(); // key
+      POP(); // on
       PUSH(value);
 
+      CHECK_ERROR();
       DISPATCH();
     }
 
@@ -924,40 +941,60 @@ PkInterpretResult vmRunScript(PKVM* vm, Script* _script) {
 
     OPCODE(ADD):
     {
-      Var r = POP(), l = POP();
-      PUSH(varAdd(vm, l, r));
+      // Don't pop yet, we need the reference for gc.
+      Var r = PEEK(-1), l = PEEK(-2);
+      Var value = varAdd(vm, l, r);
+      POP(); POP(); // r, l
+      PUSH(value);
+
       CHECK_ERROR();
       DISPATCH();
     }
 
     OPCODE(SUBTRACT):
     {
-      Var r = POP(), l = POP();
-      PUSH(varSubtract(vm, l, r));
+      // Don't pop yet, we need the reference for gc.
+      Var r = PEEK(-1), l = PEEK(-2);
+      Var value = varSubtract(vm, l, r);
+      POP(); POP(); // r, l
+      PUSH(value);
+
       CHECK_ERROR();
       DISPATCH();
     }
 
     OPCODE(MULTIPLY):
     {
-      Var r = POP(), l = POP();
-      PUSH(varMultiply(vm, l, r));
+      // Don't pop yet, we need the reference for gc.
+      Var r = PEEK(-1), l = PEEK(-2);
+      Var value = varMultiply(vm, l, r);
+      POP(); POP(); // r, l
+      PUSH(value);
+
       CHECK_ERROR();
       DISPATCH();
     }
 
     OPCODE(DIVIDE):
     {
-      Var r = POP(), l = POP();
-      PUSH(varDivide(vm, l, r));
+      // Don't pop yet, we need the reference for gc.
+      Var r = PEEK(-1), l = PEEK(-2);
+      Var value = varMultiply(vm, l, r);
+      POP(); POP(); // r, l
+      PUSH(value);
+
       CHECK_ERROR();
       DISPATCH();
     }
 
     OPCODE(MOD):
     {
-      Var r = POP(), l = POP();
-      PUSH(varModulo(vm, l, r));
+      // Don't pop yet, we need the reference for gc.
+      Var r = PEEK(-1), l = PEEK(-2);
+      Var value = varModulo(vm, l, r);
+      POP(); POP(); // r, l
+      PUSH(value);
+
       CHECK_ERROR();
       DISPATCH();
     }
@@ -985,7 +1022,7 @@ PkInterpretResult vmRunScript(PKVM* vm, Script* _script) {
     OPCODE(LT):
     {
       Var r = POP(), l = POP();
-      PUSH(VAR_BOOL(varLesser(vm, l, r)));
+      PUSH(VAR_BOOL(varLesser(l, r)));
       CHECK_ERROR();
       DISPATCH();
     }
@@ -993,7 +1030,7 @@ PkInterpretResult vmRunScript(PKVM* vm, Script* _script) {
     OPCODE(LTEQ):
     {
       Var r = POP(), l = POP();
-      bool lteq = varLesser(vm, l, r);
+      bool lteq = varLesser(l, r);
       CHECK_ERROR();
 
       if (!lteq) {
@@ -1008,7 +1045,7 @@ PkInterpretResult vmRunScript(PKVM* vm, Script* _script) {
     OPCODE(GT):
     {
       Var r = POP(), l = POP();
-      PUSH(VAR_BOOL(varGreater(vm, l, r)));
+      PUSH(VAR_BOOL(varGreater(l, r)));
       CHECK_ERROR();
       DISPATCH();
     }
@@ -1016,7 +1053,7 @@ PkInterpretResult vmRunScript(PKVM* vm, Script* _script) {
     OPCODE(GTEQ):
     {
       Var r = POP(), l = POP();
-      bool gteq = varGreater(vm, l, r);
+      bool gteq = varGreater(l, r);
       CHECK_ERROR();
 
       if (!gteq) {
@@ -1030,11 +1067,13 @@ PkInterpretResult vmRunScript(PKVM* vm, Script* _script) {
 
     OPCODE(RANGE):
     {
-      Var to = POP();
-      Var from = POP();
+      Var to = PEEK(-1);   // Don't pop yet, we need the reference for gc.
+      Var from = PEEK(-2); // Don't pop yet, we need the reference for gc.
       if (!IS_NUM(from) || !IS_NUM(to)) {
         RUNTIME_ERROR(newString(vm, "Range arguments must be number."));
       }
+      POP(); // to
+      POP(); // from
       PUSH(VAR_OBJ(newRange(vm, AS_NUM(from), AS_NUM(to))));
       DISPATCH();
     }
diff --git a/test/benchmark/fib/fib.pk b/test/benchmark/fib/fib.pk
index ca5aac3..e46ffd0 100644
--- a/test/benchmark/fib/fib.pk
+++ b/test/benchmark/fib/fib.pk
@@ -2,7 +2,7 @@
 from lang import clock
 
 def fib(n)
-  if n < 2 then return 1 end
+  if n < 2 then return n end
   return fib(n - 1) + fib(n - 2)
 end
 
diff --git a/test/benchmark/fib/fib.py b/test/benchmark/fib/fib.py
index 768ac58..dece963 100644
--- a/test/benchmark/fib/fib.py
+++ b/test/benchmark/fib/fib.py
@@ -1,7 +1,7 @@
 import time
 
 def fib(n):
-  if n < 2: return 1
+  if n < 2: return n
   return fib(n - 1) + fib(n - 2)
 
 start = time.process_time()
diff --git a/test/benchmark/fib/fib.rb b/test/benchmark/fib/fib.rb
index 96a3009..61cde40 100644
--- a/test/benchmark/fib/fib.rb
+++ b/test/benchmark/fib/fib.rb
@@ -1,7 +1,7 @@
 
 def fib(n)
   if n < 2 then
-    1
+    n
   else
     fib(n - 1) + fib(n - 2)
   end
diff --git a/test/benchmark/fib/fib.wren b/test/benchmark/fib/fib.wren
new file mode 100644
index 0000000..62adee7
--- /dev/null
+++ b/test/benchmark/fib/fib.wren
@@ -0,0 +1,12 @@
+class Fib {
+  static get(n) {
+    if (n < 2) return n
+    return get(n - 1) + get(n - 2)
+  }
+}
+
+var start = System.clock
+for (i in 0..10) {
+  System.print(Fib.get(28))
+}
+System.print("elapsed: %(System.clock - start)")
\ No newline at end of file
diff --git a/test/benchmark/for/for.js b/test/benchmark/for/for.js
new file mode 100644
index 0000000..f0ea1f0
--- /dev/null
+++ b/test/benchmark/for/for.js
@@ -0,0 +1,10 @@
+var start = +new Date();
+
+list = []
+for (var i = 0; i < 10000000; i++) { list.push(i) }
+sum = 0
+for (var i = 0; i < list.length; i++) { sum += list[i]; }
+console.log(sum)
+
+var end = +new Date();
+console.log('elapsed: ' + (end - start)/1000 + 's');
\ No newline at end of file
diff --git a/test/benchmark/for/for.pk b/test/benchmark/for/for.pk
new file mode 100644
index 0000000..fced6ec
--- /dev/null
+++ b/test/benchmark/for/for.pk
@@ -0,0 +1,11 @@
+from lang import clock
+
+start = clock()
+
+list = []
+for i in 0..10000000 do list_append(list, i) end
+sum = 0
+for i in list do sum += i end
+print(sum)
+
+print("elapsed:", clock() - start)
diff --git a/test/benchmark/for/for.py b/test/benchmark/for/for.py
new file mode 100644
index 0000000..cc8c9ee
--- /dev/null
+++ b/test/benchmark/for/for.py
@@ -0,0 +1,11 @@
+from time import process_time as clock
+
+start = clock()
+
+list = []
+for i in range(10000000): list.append(i)
+sum = 0
+for i in list: sum += i
+print(sum)
+
+print("elapsed:", clock() - start)
diff --git a/test/benchmark/for/for.rb b/test/benchmark/for/for.rb
new file mode 100644
index 0000000..d8fc32c
--- /dev/null
+++ b/test/benchmark/for/for.rb
@@ -0,0 +1,10 @@
+
+start = Time.now
+
+list = []
+for i in 0...10000000 do list.append(i) end
+sum = 0
+for i in list do sum += i end
+puts sum
+
+puts "elapsed: " + (Time.now - start).to_s + ' s'
diff --git a/test/benchmark/for/for.wren b/test/benchmark/for/for.wren
new file mode 100644
index 0000000..1ce8bd5
--- /dev/null
+++ b/test/benchmark/for/for.wren
@@ -0,0 +1,10 @@
+
+var start = System.clock
+
+var list = []
+for (i in 0...10000000) list.add(i)
+var sum = 0
+for (i in list) sum = sum + i
+System.print(sum)
+
+System.print("elapsed: %(System.clock - start)")
\ No newline at end of file