diff --git a/include/pocketpy/vm.h b/include/pocketpy/vm.h
index 3ae31b3d..47a08d1e 100644
--- a/include/pocketpy/vm.h
+++ b/include/pocketpy/vm.h
@@ -168,13 +168,13 @@ public:
     VM(bool enable_os=true);
 
     void set_main_argv(int argc, char** argv);
-    void _breakpoint();
+    void __breakpoint();
 
     Frame* top_frame(){
         return &callstack.top();
     }
 
-    void _pop_frame(){
+    void __pop_frame(){
         s_data.reset(callstack.top()._sp_base);
         callstack.pop();
 
@@ -206,14 +206,14 @@ public:
     template<typename ...Args>
     PyObject* _exec(Args&&... args){
         callstack.emplace(s_data._sp, std::forward<Args>(args)...);
-        return _run_top_frame();
+        return __run_top_frame();
     }
 
-    void _push_varargs(){}
-    void _push_varargs(PyObject* _0){ PUSH(_0); }
-    void _push_varargs(PyObject* _0, PyObject* _1){ PUSH(_0); PUSH(_1); }
-    void _push_varargs(PyObject* _0, PyObject* _1, PyObject* _2){ PUSH(_0); PUSH(_1); PUSH(_2); }
-    void _push_varargs(PyObject* _0, PyObject* _1, PyObject* _2, PyObject* _3){ PUSH(_0); PUSH(_1); PUSH(_2); PUSH(_3); }
+    void __push_varargs(){}
+    void __push_varargs(PyObject* _0){ PUSH(_0); }
+    void __push_varargs(PyObject* _0, PyObject* _1){ PUSH(_0); PUSH(_1); }
+    void __push_varargs(PyObject* _0, PyObject* _1, PyObject* _2){ PUSH(_0); PUSH(_1); PUSH(_2); }
+    void __push_varargs(PyObject* _0, PyObject* _1, PyObject* _2, PyObject* _3){ PUSH(_0); PUSH(_1); PUSH(_2); PUSH(_3); }
 
     virtual void stdout_write(const Str& s){
         _stdout(s.data, s.size);
@@ -227,7 +227,7 @@ public:
     PyObject* call(PyObject* callable, Args&&... args){
         PUSH(callable);
         PUSH(PY_NULL);
-        _push_varargs(args...);
+        __push_varargs(args...);
         return vectorcall(sizeof...(args));
     }
 
@@ -235,7 +235,7 @@ public:
     PyObject* call_method(PyObject* self, PyObject* callable, Args&&... args){
         PUSH(callable);
         PUSH(self);
-        _push_varargs(args...);
+        __push_varargs(args...);
         return vectorcall(sizeof...(args));
     }
 
@@ -258,7 +258,6 @@ public:
     void bind__hash__(Type type, i64 (*f)(VM* vm, PyObject*));
     void bind__len__(Type type, i64 (*f)(VM* vm, PyObject*));
 
-
     void bind__eq__(Type type, BinaryFuncC f);
     void bind__lt__(Type type, BinaryFuncC f);
     void bind__le__(Type type, BinaryFuncC f);
@@ -310,13 +309,13 @@ public:
     i64 normalized_index(i64 index, int size);
     PyObject* py_next(PyObject*);
     PyObject* _py_next(const PyTypeInfo*, PyObject*);
-    PyObject* _pack_next_retval(unsigned);
+    PyObject* __pack_next_retval(unsigned);
     bool py_callable(PyObject* obj);
 
-    PyObject* _minmax_reduce(bool (VM::*op)(PyObject*, PyObject*), PyObject* args, PyObject* key);
+    PyObject* __minmax_reduce(bool (VM::*op)(PyObject*, PyObject*), PyObject* args, PyObject* key);
     
     /***** Error Reporter *****/
-    void _raise(bool re_raise=false);
+    void __raise(bool re_raise=false);
 
     void _builtin_error(StrName type);
     void _builtin_error(StrName type, PyObject* arg);
@@ -411,7 +410,6 @@ public:
     void delattr(PyObject* obj, StrName name);
     PyObject* get_unbound_method(PyObject* obj, StrName name, PyObject** self, bool throw_err=true, bool fallback=false);
     void parse_int_slice(const Slice& s, int length, int& start, int& stop, int& step);
-    PyObject* _format_string(Str, PyObject*);
     void setattr(PyObject* obj, StrName name, PyObject* value);
     template<int ARGC>
     PyObject* bind_method(Type, StrName, NativeFuncC);
@@ -420,11 +418,12 @@ public:
     template<int ARGC>
     PyObject* bind_func(PyObject*, StrName, NativeFuncC, UserData userdata={}, BindType bt=BindType::DEFAULT);
     void _error(PyObject*);
-    PyObject* _run_top_frame();
+    PyObject* __run_top_frame();
     void post_init();
-    PyObject* _py_generator(Frame&& frame, ArgsView buffer);
-    void _op_unpack_sequence(uint16_t arg);
-    void _prepare_py_call(PyObject**, ArgsView, ArgsView, const FuncDecl_&);
+    PyObject* __format_string(Str, PyObject*);
+    PyObject* __py_generator(Frame&& frame, ArgsView buffer);
+    void __op_unpack_sequence(uint16_t arg);
+    void __prepare_py_call(PyObject**, ArgsView, ArgsView, const FuncDecl_&);
     // new style binding api
     PyObject* bind(PyObject*, const char*, const char*, NativeFuncC, UserData userdata={}, BindType bt=BindType::DEFAULT);
     PyObject* bind(PyObject*, const char*, NativeFuncC, UserData userdata={}, BindType bt=BindType::DEFAULT);
diff --git a/src/ceval.cpp b/src/ceval.cpp
index 15d78f0d..005c02f4 100644
--- a/src/ceval.cpp
+++ b/src/ceval.cpp
@@ -35,7 +35,7 @@ namespace pkpy{
         }
 
 
-void VM::_op_unpack_sequence(uint16_t arg){
+void VM::__op_unpack_sequence(uint16_t arg){
     PyObject* _0 = POPX();
     if(is_type(_0, VM::tp_tuple)){
         // fast path for tuple
@@ -80,14 +80,14 @@ bool VM::py_ge(PyObject* _0, PyObject* _1){
 
 #undef BINARY_F_COMPARE
 
-PyObject* VM::_run_top_frame(){
+PyObject* VM::__run_top_frame(){
     Frame* frame = top_frame();
     const Frame* base_frame = frame;
     bool need_raise = false;
 
     while(true){
         try{
-            if(need_raise){ need_raise = false; _raise(); }
+            if(need_raise){ need_raise = false; __raise(); }
 /**********************************************************************/
 /* NOTE: 
  * Be aware of accidental gc!
@@ -709,7 +709,7 @@ __NEXT_STEP:;
     } DISPATCH();
     case OP_RETURN_VALUE:{
         PyObject* _0 = byte.arg == BC_NOARG ? POPX() : None;
-        _pop_frame();
+        __pop_frame();
         if(frame == base_frame){       // [ frameBase<- ]
             return _0;
         }else{
@@ -798,7 +798,7 @@ __NEXT_STEP:;
                 frame->loop_break(&s_data, co);
             }else if(n == 1){
                 // UNPACK_SEQUENCE
-                _op_unpack_sequence(byte.arg);
+                __op_unpack_sequence(byte.arg);
             }else{
                 if(n != byte.arg){
                     ValueError(_S("expected ", (int)byte.arg, " values to unpack, got ", (int)n));
@@ -810,7 +810,7 @@ __NEXT_STEP:;
             if(_0 != StopIteration){
                 PUSH(_0);
                 // UNPACK_SEQUENCE
-                _op_unpack_sequence(byte.arg);
+                __op_unpack_sequence(byte.arg);
             }else{
                 frame->loop_break(&s_data, co);
             }
@@ -845,7 +845,7 @@ __NEXT_STEP:;
     } DISPATCH();
     /*****************************************/
     case OP_UNPACK_SEQUENCE:{
-        _op_unpack_sequence(byte.arg);
+        __op_unpack_sequence(byte.arg);
     } DISPATCH();
     case OP_UNPACK_EX: {
         auto _lock = heap.gc_scope_lock();  // lock the gc via RAII!!
@@ -939,13 +939,13 @@ __NEXT_STEP:;
             _builtin_error("AssertionError");
         }
         DISPATCH();
-    case OP_RE_RAISE: _raise(true); DISPATCH();
+    case OP_RE_RAISE: __raise(true); DISPATCH();
     case OP_POP_EXCEPTION: _last_exception = POPX(); DISPATCH();
     /*****************************************/
     case OP_FORMAT_STRING: {
         PyObject* _0 = POPX();
         const Str& spec = CAST(Str&, co->consts[byte.arg]);
-        PUSH(_format_string(spec, _0));
+        PUSH(__format_string(spec, _0));
     } DISPATCH();
     /*****************************************/
     case OP_INC_FAST:{
@@ -981,7 +981,7 @@ __NEXT_STEP:;
             PyObject* e_obj = POPX();
             Exception& _e = PK_OBJ_GET(Exception, e_obj);
             bool is_base_frame_to_be_popped = frame == base_frame;
-            _pop_frame();
+            __pop_frame();
             if(callstack.empty()) throw _e;   // propagate to the top level
             frame = top_frame();
             PUSH(e_obj);
diff --git a/src/iter.cpp b/src/iter.cpp
index c8ed0383..be332e36 100644
--- a/src/iter.cpp
+++ b/src/iter.cpp
@@ -56,7 +56,7 @@ namespace pkpy{
 
         PyObject* ret;
         try{
-            ret = vm->_run_top_frame();
+            ret = vm->__run_top_frame();
         }catch(...){
             state = 2;      // end this generator immediately when an exception is thrown
             throw;
@@ -67,7 +67,7 @@ namespace pkpy{
             frame = std::move(vm->callstack.top());
             ret = vm->s_data.popx();
             for(PyObject* obj: frame.stack_view(&vm->s_data)) s_backup.push_back(obj);
-            vm->_pop_frame();
+            vm->__pop_frame();
             state = 1;
             if(ret == vm->StopIteration) state = 2;
             return ret;
@@ -103,7 +103,7 @@ namespace pkpy{
         });
     }
 
-PyObject* VM::_py_generator(Frame&& frame, ArgsView buffer){
+PyObject* VM::__py_generator(Frame&& frame, ArgsView buffer){
     return vm->new_user_object<Generator>(std::move(frame), buffer);
 }
 
diff --git a/src/pocketpy.cpp b/src/pocketpy.cpp
index ceee885e..855f4da3 100644
--- a/src/pocketpy.cpp
+++ b/src/pocketpy.cpp
@@ -163,11 +163,11 @@ void init_builtins(VM* _vm) {
     });
 
     _vm->bind(_vm->builtins, "max(*args, key=None)", [](VM* vm, ArgsView args){
-        return vm->_minmax_reduce(&VM::py_gt, args[0], args[1]);
+        return vm->__minmax_reduce(&VM::py_gt, args[0], args[1]);
     });
 
     _vm->bind(_vm->builtins, "min(*args, key=None)", [](VM* vm, ArgsView args){
-        return vm->_minmax_reduce(&VM::py_lt, args[0], args[1]);
+        return vm->__minmax_reduce(&VM::py_lt, args[0], args[1]);
     });
 
     _vm->bind_func<1>(_vm->builtins, "id", [](VM* vm, ArgsView args) {
diff --git a/src/vm.cpp b/src/vm.cpp
index b5ab9430..ef7a8de4 100644
--- a/src/vm.cpp
+++ b/src/vm.cpp
@@ -249,7 +249,7 @@ namespace pkpy{
     PyObject* VM::_py_next(const PyTypeInfo* ti, PyObject* obj){
         if(ti->m__next__){
             unsigned n = ti->m__next__(this, obj);
-            return _pack_next_retval(n);
+            return __pack_next_retval(n);
         }
         return call_method(obj, __next__);
     }
@@ -270,7 +270,7 @@ namespace pkpy{
         return vm->find_name_in_mro(cls, __call__) != nullptr;
     }
 
-    PyObject* VM::_minmax_reduce(bool (VM::*op)(PyObject*, PyObject*), PyObject* args, PyObject* key){
+    PyObject* VM::__minmax_reduce(bool (VM::*op)(PyObject*, PyObject*), PyObject* args, PyObject* key){
         auto _lock = heap.gc_scope_lock();
         const Tuple& args_tuple = PK_OBJ_GET(Tuple, args);  // from *args, it must be a tuple
         if(key==vm->None && args_tuple.size()==2){
@@ -499,7 +499,7 @@ i64 VM::py_hash(PyObject* obj){
     }
 }
 
-PyObject* VM::_format_string(Str spec, PyObject* obj){
+PyObject* VM::__format_string(Str spec, PyObject* obj){
     if(spec.empty()) return py_str(obj);
     char type;
     switch(spec.end()[-1]){
@@ -844,7 +844,7 @@ void VM::_unpack_as_dict(ArgsView args, Dict& dict){
 }
 
 
-void VM::_prepare_py_call(PyObject** buffer, ArgsView args, ArgsView kwargs, const FuncDecl_& decl){
+void VM::__prepare_py_call(PyObject** buffer, ArgsView args, ArgsView kwargs, const FuncDecl_& decl){
     const CodeObject* co = decl->code.get();
     int co_nlocals = co->varnames.size();
     int decl_argc = decl->args.size();
@@ -943,7 +943,7 @@ PyObject* VM::vectorcall(int ARGC, int KWARGC, bool op_call){
         switch(fn.decl->type){
             case FuncType::UNSET: PK_FATAL_ERROR(); break;
             case FuncType::NORMAL:
-                _prepare_py_call(buffer, args, kwargs, fn.decl);
+                __prepare_py_call(buffer, args, kwargs, fn.decl);
                 // copy buffer back to stack
                 s_data.reset(_base + co_nlocals);
                 for(int j=0; j<co_nlocals; j++) _base[j] = buffer[j];
@@ -963,9 +963,9 @@ PyObject* VM::vectorcall(int ARGC, int KWARGC, bool op_call){
                 s_data.reset(p0);
                 return None;
             case FuncType::GENERATOR:
-                _prepare_py_call(buffer, args, kwargs, fn.decl);
+                __prepare_py_call(buffer, args, kwargs, fn.decl);
                 s_data.reset(p0);
-                return _py_generator(
+                return __py_generator(
                     Frame(nullptr, co, fn._module, callable, nullptr),
                     ArgsView(buffer, buffer + co_nlocals)
                 );
@@ -974,7 +974,7 @@ PyObject* VM::vectorcall(int ARGC, int KWARGC, bool op_call){
         // simple or normal
         callstack.emplace(p0, co, fn._module, callable, args.begin());
         if(op_call) return PY_OP_CALL;
-        return _run_top_frame();
+        return __run_top_frame();
         /*****************_py_call*****************/
     }
 
@@ -983,7 +983,7 @@ PyObject* VM::vectorcall(int ARGC, int KWARGC, bool op_call){
         PyObject* ret;
         if(f.decl != nullptr){
             int co_nlocals = f.decl->code->varnames.size();
-            _prepare_py_call(buffer, args, kwargs, f.decl);
+            __prepare_py_call(buffer, args, kwargs, f.decl);
             // copy buffer back to stack
             s_data.reset(_base + co_nlocals);
             for(int j=0; j<co_nlocals; j++) _base[j] = buffer[j];
@@ -1294,10 +1294,10 @@ void VM::_error(PyObject* e_obj){
         throw e;
     }
     PUSH(e_obj);
-    _raise();
+    __raise();
 }
 
-void VM::_raise(bool re_raise){
+void VM::__raise(bool re_raise){
     Frame* frame = top_frame();
     Exception& e = PK_OBJ_GET(Exception, s_data.top());
     if(!re_raise){
@@ -1365,7 +1365,7 @@ void VM::bind__delitem__(Type type, void (*f)(VM*, PyObject*, PyObject*)){
 }
 
 
-    PyObject* VM::_pack_next_retval(unsigned n){
+    PyObject* VM::__pack_next_retval(unsigned n){
         if(n == 0) return StopIteration;
         if(n == 1) return s_data.popx();
         PyObject* retval = VAR(s_data.view(n).to_tuple());
@@ -1377,7 +1377,7 @@ void VM::bind__delitem__(Type type, void (*f)(VM*, PyObject*, PyObject*)){
         _all_types[type].m__next__ = f;                                                   \
         PyObject* nf = bind_method<0>(_t(type), __next__, [](VM* vm, ArgsView args){       \
             int n = lambda_get_userdata<unsigned(*)(VM*, PyObject*)>(args.begin())(vm, args[0]);\
-            return vm->_pack_next_retval(n);                                               \
+            return vm->__pack_next_retval(n);                                               \
         });                                                                             \
         PK_OBJ_GET(NativeFunc, nf).set_userdata(f);                                        \
     }
@@ -1500,20 +1500,20 @@ void NextBreakpoint::_step(VM* vm){
     int curr_lineno = vm->top_frame()->curr_lineno();
     if(should_step_into){
         if(curr_callstack_size != callstack_size || curr_lineno != lineno){
-            vm->_breakpoint();
+            vm->__breakpoint();
         }
     }else{
         if(curr_callstack_size == callstack_size) {
-            if(curr_lineno != lineno) vm->_breakpoint();
+            if(curr_lineno != lineno) vm->__breakpoint();
         }else if(curr_callstack_size < callstack_size){
             // returning
-            vm->_breakpoint();
+            vm->__breakpoint();
         }
     }
 }
 #endif
 
-void VM::_breakpoint(){
+void VM::__breakpoint(){
 #if PK_ENABLE_PROFILER
     _next_breakpoint = NextBreakpoint();