diff --git a/nasal_dbg.h b/nasal_dbg.h
index 2d2a197..88f529a 100644
--- a/nasal_dbg.h
+++ b/nasal_dbg.h
@@ -301,14 +301,12 @@ void nasal_dbg::run(
         ++count[code[pc]];
         (this->*oprs[code[pc]])();
         if(top>=canary)
-            break;
+            die("stack overflow");
         ++pc;
     }
 #endif
 
 vmexit:
-    if(top>=canary)
-        die("stack overflow");
     callsort(count);
     gc.clear();
     imm.clear();
@@ -316,12 +314,14 @@ vmexit:
     return;
 #ifndef _MSC_VER
 #define dbg(op,num) {\
-    interact();\
-    op();\
-    ++count[num];\
-    if(top<canary)\
+        interact();\
+        op();\
+        ++count[num];\
+        if(top<canary)\
+            goto *code[++pc];\
+        die("stack overflow");\
         goto *code[++pc];\
-    goto vmexit;}
+    }
 
 intg:   dbg(o_intg  ,op_intg  );
 intl:   dbg(o_intl  ,op_intl  );
diff --git a/nasal_gc.h b/nasal_gc.h
index 6f709e9..663e1f8 100644
--- a/nasal_gc.h
+++ b/nasal_gc.h
@@ -76,10 +76,11 @@ struct nas_ref
     nas_ref(const u8 t,const i64 n):type(t){val.cnt=n;}
     // vm_num
     nas_ref(const u8 t,const f64 n):type(t){val.num=n;}
-    // vm_str/vm_func/vm_vec/vm_hash/vm_upval/vm_obj
+    // nas_val
     nas_ref(const u8 t,nas_val* n):type(t){val.gcobj=n;}
     // vm_addr
     nas_ref(const u8 t,nas_ref* n):type(t){val.addr=n;}
+    // copy
     nas_ref(const nas_ref& nr):type(nr.type),val(nr.val){}
     bool operator==(const nas_ref& nr){return type==nr.type && val.gcobj==nr.val.gcobj;}
     bool operator!=(const nas_ref& nr){return type!=nr.type || val.gcobj!=nr.val.gcobj;}
diff --git a/nasal_vm.h b/nasal_vm.h
index a69105a..be3cfdc 100644
--- a/nasal_vm.h
+++ b/nasal_vm.h
@@ -1035,6 +1035,7 @@ vmexit:
         if(top<canary)\
             goto *code[++pc];\
         die("stack overflow");\
+        goto *code[++pc];\
     }
 // do not cause stackoverflow
 #define exec_nodie(op) {op();goto *code[++pc];}
diff --git a/stl/json.nas b/stl/json.nas
new file mode 100644
index 0000000..e490022
--- /dev/null
+++ b/stl/json.nas
@@ -0,0 +1,268 @@
+# lib json.nas
+# 2021 ValKmjolnir
+var JSON=func(){
+
+    var (
+        j_eof,
+        j_lbrace,
+        j_rbrace,
+        j_lbracket,
+        j_rbracket,
+        j_comma,
+        j_colon,
+        j_str,
+        j_num,
+        j_id
+    )=(0,1,2,3,4,5,6,7,8,9);
+    var j_content=[
+        "eof",
+        "`{`",
+        "`}`",
+        "`[`",
+        "`]`",
+        "`,`",
+        "`:`",
+        "string",
+        "number",
+        "identifier"
+    ];
+
+    var text='';
+    var line=1;
+    var text_size=0;
+    var ptr=0;
+    var token={content:'',type:''};
+    var content={};
+    var init=func(){
+        text='';
+        line=1;
+        text_size=0;
+        ptr=0;
+        content={};
+        token={content:'',type:''};
+    }
+
+    var isnum=func(c){
+        return '0'<=c and c<='9';
+    }
+    var isid=func(c){
+        var tmp=c[0];
+        return ('a'[0]<=tmp and tmp<='z'[0]) or
+        ('A'[0]<=tmp and tmp<='Z'[0]) or
+        c=='_';
+    }
+    var check=func(){
+        var c=text[ptr];
+        return (
+            c=='{' or c=='}' or
+            c=='[' or c==']' or
+            c==',' or c==':' or
+            c=='\"' or c=='\'' or
+            isnum(c) or isid(c)
+        );
+    }
+
+    var get=func(str){
+        init();
+        if(!size(str))
+            die("empty string");
+        text=split('',str);
+        text_size=size(text);
+        return;
+    }
+    var next=func(){
+        while(ptr<text_size and !check()){
+            if(text[ptr]=='\n')
+                line+=1;
+            ptr+=1;
+        }
+        if(ptr>=text_size){
+            token.content="eof";
+            token.type=j_eof;
+            return;
+        }
+        
+        var c=text[ptr];
+        if(c=='{'){
+            token.content='{';
+            token.type=j_lbrace;
+        }elsif(c=='}'){
+            token.content='}';
+            token.type=j_rbrace;
+        }elsif(c=='['){
+            token.content='[';
+            token.type=j_lbracket;
+        }elsif(c==']'){
+            token.content=']';
+            token.type=j_rbracket;
+        }elsif(c==','){
+            token.content=',';
+            token.type=j_comma;
+        }elsif(c==':'){
+            token.content=':';
+            token.type=j_colon;
+        }elsif(c=='\"' or c=='\''){
+            var strbegin=c;
+            var s="";
+            ptr+=1;
+            while(ptr<text_size and text[ptr]!=strbegin){
+                s~=text[ptr];
+                ptr+=1;
+            }
+            token.content=s;
+            token.type=j_str;
+        }elsif(isnum(c)){
+            var s=c;
+            ptr+=1;
+            while(ptr<text_size and ((isnum(text[ptr]) or text[ptr]=='.'))){
+                s~=text[ptr];
+                ptr+=1;
+            }
+            ptr-=1;
+            token.content=num(s);
+            token.type=j_num;
+        }elsif(isid(c)){
+            var s=c;
+            ptr+=1;
+            while(ptr<text_size and (isid(text[ptr]) or isnum(text[ptr]))){
+                s~=text[ptr];
+                ptr+=1;
+            }
+            ptr-=1;
+            token.content=s;
+            token.type=j_id;
+        }
+        ptr+=1;
+        return;
+    }
+
+    var match=func(type){
+        if(token.type!=type)
+            print("line ",line,": expect ",j_content[type]," but get `",token.content,"`.\n");
+        next();
+        return;
+    }
+
+    var hash_gen=func(){
+        var hash={};
+        match(j_lbrace);
+        member(hash);
+        while(token.type==j_comma){
+            match(j_comma);
+            member(hash);
+        }
+        match(j_rbrace);
+        return hash;
+    }
+
+    var vec_gen=func(){
+        var vec=[];
+        match(j_lbracket);
+        if(token.type==j_lbrace){
+            append(vec,hash_gen());
+        }elsif(token.type==j_lbracket){
+            append(vec,vec_gen());
+        }elsif(token.type==j_str or token.type==j_num){
+            append(vec,token.content);
+            next();
+        }
+        while(token.type==j_comma){
+            match(j_comma);
+            if(token.type==j_lbrace){
+                append(vec,me.hash_gen());
+            }elsif(token.type==j_lbracket){
+                append(vec,vec_gen());
+            }elsif(token.type==j_str or token.type==j_num){
+                append(vec,token.content);
+                next();
+            }
+        }
+        match(j_rbracket);
+        return vec;
+    }
+
+    var member=func(hash){
+        var name=token.content;
+        if(token.type==j_rbrace){
+            return;
+        }
+        if(token.type==j_str){
+            match(j_str);
+        }else{
+            match(j_id);
+        }
+        match(j_colon);
+        if(token.type==j_lbrace){
+            hash[name]=hash_gen();
+        }elsif(token.type==j_lbracket){
+            hash[name]=vec_gen();
+        }elsif(token.type==j_str or token.type==j_num){
+            hash[name]=token.content;
+            next();
+        }
+        return;
+    }
+    return {
+        parse:func(str){
+            if(typeof(str)!="str")
+                die("JSON.parse: must use string");
+            get(str);
+            next();
+
+            match(j_lbrace);
+            member(content);
+            while(token.type==j_comma){
+                match(j_comma);
+                member(content);
+            }
+            match(j_rbrace);
+
+            var res=content;
+            init();
+            return res;
+        },
+        stringify:func(hash){
+            if(typeof(hash)!="hash")
+                die("JSON.stringify: must use hashmap");
+            var s="";
+            var gen=func(elem){
+                var t=typeof(elem);
+                if(t=="num"){
+                    s~=elem;
+                }elsif(t=="str"){
+                    s~='"'~elem~'"';
+                }elsif(t=="vec"){
+                    vgen(elem);
+                }elsif(t=="hash"){
+                    hgen(elem);
+                }else{
+                    s~='"undefined"';
+                }
+            }
+            var vgen=func(v){
+                s~="[";
+                var vsize=size(v);
+                for(var i=0;i<vsize;i+=1){
+                    gen(v[i]);
+                    if(i!=vsize-1)
+                        s~=",";
+                }
+                s~="]";
+            }
+            var hgen=func(h){
+                s~="{";
+                var k=keys(h);
+                var vsize=size(k);
+                for(var i=0;i<vsize;i+=1){
+                    s~=k[i]~":";
+                    gen(h[k[i]]);
+                    if(i!=vsize-1)
+                        s~=",";
+                }
+                s~="}";
+            }
+            hgen(hash);
+            return s;
+        }
+    };
+}();
diff --git a/test/calc.nas b/test/calc.nas
index b910b20..d48b270 100644
--- a/test/calc.nas
+++ b/test/calc.nas
@@ -19,6 +19,7 @@ var source=[
 var lib=[
     "fg_env.nas",
     "file.nas",
+    "json.nas",
     "lib.nas",
     "list.nas",
     "log.nas",
diff --git a/test/coroutine.nas b/test/coroutine.nas
index a62840a..dc06aa8 100644
--- a/test/coroutine.nas
+++ b/test/coroutine.nas
@@ -35,6 +35,38 @@ func(){
     }
 }();
 
+# test crash in coroutines
+var co=coroutine.create(func{
+    var b=func(){b()}
+    coroutine.yield(b);
+    b();
+    coroutine.yield(0);
+});
+
+println("coroutine yield: ",coroutine.resume(co));
+println("coroutine state:\e[32m ",coroutine.status(co),"\e[0m");
+println("coroutine error: ",coroutine.resume(co));
+println("coroutine state:\e[91m ",coroutine.status(co),"\e[0m");
+println("coroutine yield: ",coroutine.resume(co));
+println("coroutine state:\e[91m ",coroutine.status(co),"\e[0m");
+
+var co=coroutine.create(func{
+    var a=1;
+    var b=func(){
+        b();
+    }
+    coroutine.yield(b);
+    coroutine.yield(b());
+});
+
+println("coroutine yield: ",coroutine.resume(co));
+println("coroutine state:\e[32m ",coroutine.status(co),"\e[0m");
+println("coroutine error: ",coroutine.resume(co));
+println("coroutine state:\e[91m ",coroutine.status(co),"\e[0m");
+println("coroutine yield: ",coroutine.resume(co));
+println("coroutine state:\e[91m ",coroutine.status(co),"\e[0m");
+println("ok");
+
 # pressure test
 var productor=func(){
     for(var i=0;;i+=1)
@@ -62,17 +94,3 @@ tm.stamp();
 while(tm.elapsedMSec()<total)
     consumer();
 println("\nexecute ",counter," tasks during ",total," ms, avg ",counter/total," tasks/ms.");
-
-var co=coroutine.create(func{
-    var b=func(){b()}
-    coroutine.yield(b);
-    b();
-    coroutine.yield(0);
-});
-
-println("coroutine yield: ",coroutine.resume(co));
-println("coroutine state:\e[32m ",coroutine.status(co),"\e[0m");
-println("coroutine error: ",coroutine.resume(co));
-println("coroutine state:\e[91m ",coroutine.status(co),"\e[0m");
-println("coroutine yield: ",coroutine.resume(co));
-println("coroutine state:\e[91m ",coroutine.status(co),"\e[0m");
\ No newline at end of file
diff --git a/test/json.nas b/test/json.nas
index d363aa6..7772d1c 100644
--- a/test/json.nas
+++ b/test/json.nas
@@ -1,278 +1,4 @@
-#lib json.nas
-var JSON=func(){
-
-    var (
-        j_eof,
-        j_lbrace,
-        j_rbrace,
-        j_lbracket,
-        j_rbracket,
-        j_comma,
-        j_colon,
-        j_str,
-        j_num,
-        j_id
-    )=(
-        0,
-        1,
-        2,
-        3,
-        4,
-        5,
-        6,
-        7,
-        8,
-        9
-    );
-    var j_content=[
-        "eof",
-        "`{`",
-        "`}`",
-        "`[`",
-        "`]`",
-        "`,`",
-        "`:`",
-        "string",
-        "number",
-        "identifier"
-    ];
-
-    var text='';
-    var line=1;
-    var text_size=0;
-    var ptr=0;
-    var token={content:'',type:''};
-    var content={};
-    var init=func(){
-        line=1;
-        text_size=0;
-        ptr=0;
-        content={};
-        token={content:'',type:''};
-        text='';
-    }
-
-    var isnum=func(c){
-        return '0'<=c and c<='9';
-    }
-    var isid=func(c){
-        var tmp=c[0];
-        return ('a'[0]<=tmp and tmp<='z'[0]) or
-        ('A'[0]<=tmp and tmp<='Z'[0]) or
-        c=='_';
-    }
-    var check=func(){
-        var c=text[ptr];
-        return (
-            c=='{' or c=='}' or
-            c=='[' or c==']' or
-            c==',' or c==':' or
-            c=='\"' or c=='\'' or
-            isnum(c) or isid(c)
-        );
-    }
-
-    var get=func(str){
-        init();
-        if(!size(str))
-            die("empty string");
-        text=split('',str);
-        text_size=size(text);
-        return;
-    }
-    var next=func(){
-        while(ptr<text_size and !check()){
-            if(text[ptr]=='\n')
-                line+=1;
-            ptr+=1;
-        }
-        if(ptr>=text_size){
-            token.content="eof";
-            token.type=j_eof;
-            return;
-        }
-        
-        var c=text[ptr];
-        if(c=='{'){
-            token.content='{';
-            token.type=j_lbrace;
-        }elsif(c=='}'){
-            token.content='}';
-            token.type=j_rbrace;
-        }elsif(c=='['){
-            token.content='[';
-            token.type=j_lbracket;
-        }elsif(c==']'){
-            token.content=']';
-            token.type=j_rbracket;
-        }elsif(c==','){
-            token.content=',';
-            token.type=j_comma;
-        }elsif(c==':'){
-            token.content=':';
-            token.type=j_colon;
-        }elsif(c=='\"' or c=='\''){
-            var strbegin=c;
-            var s="";
-            ptr+=1;
-            while(ptr<text_size and text[ptr]!=strbegin){
-                s~=text[ptr];
-                ptr+=1;
-            }
-            token.content=s;
-            token.type=j_str;
-        }elsif(isnum(c)){
-            var s=c;
-            ptr+=1;
-            while(ptr<text_size and ((isnum(text[ptr]) or text[ptr]=='.'))){
-                s~=text[ptr];
-                ptr+=1;
-            }
-            ptr-=1;
-            token.content=num(s);
-            token.type=j_num;
-        }elsif(isid(c)){
-            var s=c;
-            ptr+=1;
-            while(ptr<text_size and (isid(text[ptr]) or isnum(text[ptr]))){
-                s~=text[ptr];
-                ptr+=1;
-            }
-            ptr-=1;
-            token.content=s;
-            token.type=j_id;
-        }
-        ptr+=1;
-        return;
-    }
-
-    var match=func(type){
-        if(token.type!=type)
-            print("line ",line,": expect ",j_content[type]," but get `",token.content,"`.\n");
-        next();
-        return;
-    }
-
-    var hash_gen=func(){
-        var hash={};
-        match(j_lbrace);
-        member(hash);
-        while(token.type==j_comma){
-            match(j_comma);
-            member(hash);
-        }
-        match(j_rbrace);
-        return hash;
-    }
-
-    var vec_gen=func(){
-        var vec=[];
-        match(j_lbracket);
-        if(token.type==j_lbrace)
-            append(vec,hash_gen());
-        elsif(token.type==j_lbracket)
-            append(vec,vec_gen());
-        elsif(token.type==j_str or token.type==j_num){
-            append(vec,token.content);
-            next();
-        }
-        while(token.type==j_comma){
-            match(j_comma);
-            if(token.type==j_lbrace)
-                append(vec,me.hash_gen());
-            elsif(token.type==j_lbracket)
-                append(vec,vec_gen());
-            elsif(token.type==j_str or token.type==j_num){
-                append(vec,token.content);
-                next();
-            }
-        }
-        match(j_rbracket);
-        return vec;
-    }
-
-    var member=func(hash){
-        var name=token.content;
-        if(token.type==j_rbrace)
-            return;
-        if(token.type==j_str)
-            match(j_str);
-        else
-            match(j_id);
-        match(j_colon);
-        if(token.type==j_lbrace)
-            hash[name]=hash_gen();
-        elsif(token.type==j_lbracket)
-            hash[name]=vec_gen();
-        elsif(token.type==j_str or token.type==j_num){
-            hash[name]=token.content;
-            next();
-        }
-        return;
-    }
-    return {
-        parse:func(str){
-            if(typeof(str)!="str")
-                die("JSON.parse: must use string");
-            get(str);
-            next();
-
-            match(j_lbrace);
-            member(content);
-            while(token.type==j_comma){
-                match(j_comma);
-                member(content);
-            }
-            match(j_rbrace);
-
-            var res=content;
-            init();
-            return res;
-        },
-        stringify:func(hash){
-            if(typeof(hash)!="hash")
-                die("JSON.stringify: must use hashmap");
-            var s="";
-            var gen=func(elem){
-                var t=typeof(elem);
-                if(t=="num")
-                    s~=elem;
-                elsif(t=="str")
-                    s~='"'~elem~'"';
-                elsif(t=="vec")
-                    vgen(elem);
-                elsif(t=="hash")
-                    hgen(elem);
-                else
-                    s~='"undefined"';
-            }
-            var vgen=func(v){
-                s~="[";
-                var vsize=size(v);
-                for(var i=0;i<vsize;i+=1){
-                    gen(v[i]);
-                    if(i!=vsize-1)
-                        s~=",";
-                }
-                s~="]";
-            }
-            var hgen=func(h){
-                s~="{";
-                var k=keys(h);
-                var vsize=size(k);
-                for(var i=0;i<vsize;i+=1){
-                    s~=k[i]~":";
-                    gen(h[k[i]]);
-                    if(i!=vsize-1)
-                        s~=",";
-                }
-                s~="}";
-            }
-            hgen(hash);
-            return s;
-        }
-    };
-}();
+import.stl.json;
 
 var ss=JSON.stringify({
     vec:[0,1,2],
diff --git a/test/md5compare.nas b/test/md5compare.nas
index 172f747..bf8a075 100644
--- a/test/md5compare.nas
+++ b/test/md5compare.nas
@@ -33,6 +33,7 @@ var compare=func(){
 var filechecksum=func(){
     var files=[
         "./stl/fg_env.nas",         "./stl/file.nas",
+        "./stl/json.nas",
         "./stl/lib.nas",            "./stl/list.nas",
         "./stl/log.nas",            "./stl/module.nas",
         "./stl/padding.nas",        "./stl/process_bar.nas",