diff --git a/nasal_builtin.h b/nasal_builtin.h
index 28c2fa8..cf6149e 100644
--- a/nasal_builtin.h
+++ b/nasal_builtin.h
@@ -732,6 +732,11 @@ nasal_ref builtin_chr(nasal_ref* local,nasal_gc& gc)
         ret.str()=" ";
     return ret;
 }
+void obj_file_destructor(void* ptr)
+{
+    fclose((FILE*)ptr);
+    return;
+}
 nasal_ref builtin_open(nasal_ref* local,nasal_gc& gc)
 {
     nasal_ref filename=local[1];
@@ -746,14 +751,16 @@ nasal_ref builtin_open(nasal_ref* local,nasal_gc& gc)
     nasal_ref ret=gc.alloc(vm_obj);
     ret.obj().type=obj_file;
     ret.obj().ptr=(void*)res;
+    ret.obj().destructor=obj_file_destructor;
     return ret;
 }
 nasal_ref builtin_close(nasal_ref* local,nasal_gc& gc)
 {
-    nasal_ref filehandle=local[1];
-    if(filehandle.type!=vm_obj || filehandle.obj().type!=obj_file)
+    nasal_ref fd=local[1];
+    if(fd.type!=vm_obj || fd.obj().type!=obj_file)
         return builtin_err("close","not a correct filehandle");
-    fclose((FILE*)filehandle.obj().ptr);
+    fclose((FILE*)fd.obj().ptr);
+    fd.obj().ptr=nullptr;
     return nil;
 }
 nasal_ref builtin_read(nasal_ref* local,nasal_gc& gc)
diff --git a/nasal_gc.h b/nasal_gc.h
index 2709468..125acbe 100644
--- a/nasal_gc.h
+++ b/nasal_gc.h
@@ -148,16 +148,21 @@ struct nasal_upval
 
 struct nasal_obj
 {
+    /* RAII constructor */
+    /* new object is initialized when creating */
     uint32_t type;
     void* ptr;
-    void* destructor;
+
+    /* RAII destroyer */
+    typedef void (*dest)(void*);
+    dest destructor;
+
     nasal_obj():ptr(nullptr),destructor(nullptr){}
     ~nasal_obj(){clear();}
     void clear()
-    {
-        typedef void (*func)(void*);
-        if(destructor)
-            (func(destructor))(ptr);
+    {   
+        if(destructor && ptr)
+            destructor(ptr);
         ptr=nullptr;
     }
 };
diff --git a/test/tetris.nas b/test/tetris.nas
index d962405..5cde440 100644
--- a/test/tetris.nas
+++ b/test/tetris.nas
@@ -238,6 +238,7 @@ var mapgen=func(mapx,mapy){
     }
 
     var checkmap=func(){
+        var lines=1;
         for(var y=mapy-1;y>=0;y-=1){
             # check if this line is full of blocks
             var tmp=0;
@@ -249,7 +250,8 @@ var mapgen=func(mapx,mapy){
             # if is full, clear this line and
             # all the lines above fall one block
             if(x==mapx){
-                score+=tmp;
+                score+=lines*tmp;
+                lines*=2;
                 for(var t=y;t>=1;t-=1)
                     for(var x=0;x<mapx;x+=1)
                         map[t][x]=map[t-1][x];
@@ -296,7 +298,7 @@ var main=func(){
     libkey.getch();
     print("\ec");
 
-    var counter=15;
+    var counter=30;
     while(1){
         # nonblock input one character
         var ch=libkey.nonblock();
@@ -326,9 +328,9 @@ var main=func(){
             map.checkmap();
             if(map.gameover())
                 break;
-            counter=15;
+            counter=30;
         }
-        unix.sleep(0.04);
+        unix.sleep(0.02);
         counter-=1;
     }
     libkey.close();