diff --git a/src/include/multi_array_operation.cc.h b/src/include/multi_array_operation.cc.h
index 0f1fa41..2b554db 100644
--- a/src/include/multi_array_operation.cc.h
+++ b/src/include/multi_array_operation.cc.h
@@ -346,6 +346,7 @@ namespace MultiArrayTools
     template <typename V, class ET>
     inline const V& ConstOperationRoot<T,Ranges...>::vget(ET pos) const
     {
+	VCHECK(pos.val());
 	return *(reinterpret_cast<const V*>(mDataPtr)+pos.val());
     }
 
@@ -595,29 +596,30 @@ namespace MultiArrayTools
     template <bool VABLE = false>
     struct VExec
     {
-	template <typename TarOp, class OpClass>
+	template <template <typename> class F, typename TarOp, class OpClass>
 	static inline void exec(TarOp& th, const OpClass& in)
 	{
-	    th.assign(in)();
+	    typedef typename TarOp::value_type T;
+	    th.template asx<IAccess<F<T>>>(in)();
 	}
     };
 
     template <>
     struct VExec<true>
     {
-	template <typename TarOp, class OpClass>
+	template <template <typename> class F, typename TarOp, class OpClass>
 	static inline void exec(TarOp& th, const OpClass& in)
 	{
 	    CHECK;
 	    typedef typename TarOp::value_type T;
-	    auto x = th.template asx<IVAssign<typename VType<T>::type,T>>(in);
+	    auto x = th.template asx<IVAccess<typename VType<T>::type,F<T>>>(in);
 	    const size_t inum = x.vec(VType<T>::MULT);
 	    if(x.rootSteps(inum) == 1){
 		CHECK;
 		x();
 	    }
 	    else {
-		th.assign(in)();
+		th.template asx<IAccess<F<T>>>(in)();
 	    }
 	}
     };
@@ -626,7 +628,7 @@ namespace MultiArrayTools
     template <class OpClass>
     OperationRoot<T,Ranges...>& OperationRoot<T,Ranges...>::operator=(const OpClass& in)
     {
-	VExec<OpClass::VABLE>::exec(*this,in);
+	VExec<OpClass::VABLE>::template exec<identity>(*this,in);
         return *this;
     }
 
@@ -634,7 +636,8 @@ namespace MultiArrayTools
     template <class OpClass>
     OperationRoot<T,Ranges...>& OperationRoot<T,Ranges...>::operator+=(const OpClass& in)
     {
-        plus(in)();
+	VExec<OpClass::VABLE>::template exec<xxxplus>(*this,in);
+        //plus(in)();
         return *this;
     }
 
@@ -661,6 +664,7 @@ namespace MultiArrayTools
     template <typename V, class ET>
     inline V& OperationRoot<T,Ranges...>::vget(ET pos) const
     {
+	VCHECK(pos.val());
 	return *(reinterpret_cast<V*>(mDataPtr)+pos.val());
     }
 
@@ -732,46 +736,77 @@ namespace MultiArrayTools
     }
 
     template <typename T, class... Ranges>
-    template <class OpClass>
-    auto ParallelOperationRoot<T,Ranges...>::assign(const OpClass& in)
-        -> decltype(mIndex.pifor(1,in.loop(AssignmentExpr2<T,ParallelOperationRoot<T,Ranges...>,OpClass,OpIndexAff::TARGET>
+    template <class IOp, class OpClass>
+    auto ParallelOperationRoot<T,Ranges...>::asx(const OpClass& in) const
+        -> decltype(mIndex.pifor(1,in.loop(AssignmentExpr<T,IOp,ParallelOperationRoot<T,Ranges...>,OpClass,OpIndexAff::TARGET>
                                           (mOrigDataPtr,*this,in))))
 
     {
-        return mIndex.pifor(1,in.loop(AssignmentExpr2<T,ParallelOperationRoot<T,Ranges...>,OpClass,OpIndexAff::TARGET>
-                                      (mOrigDataPtr,*this,in)));
+        static_assert( OpClass::SIZE == decltype(in.rootSteps())::SIZE, "Ext Size mismatch" );
+        return mIndex.pifor(1,in.loop(AssignmentExpr<T,IOp,ParallelOperationRoot<T,Ranges...>,OpClass,OpIndexAff::TARGET>
+                                     (mOrigDataPtr,*this,in)));
     }
 
     template <typename T, class... Ranges>
-    template <class OpClass, class Index>
-    auto ParallelOperationRoot<T,Ranges...>::assign(const OpClass& in, const std::shared_ptr<Index>& i) const
-        -> decltype(i->pifor(1,in.loop(AssignmentExpr2<T,ParallelOperationRoot<T,Ranges...>,OpClass>
-				       (mOrigDataPtr,*this,in))))
+    template <class IOp, class OpClass>
+    auto ParallelOperationRoot<T,Ranges...>::asxExpr(const OpClass& in) const
+        -> decltype(in.loop(AssignmentExpr<T,IOp,ParallelOperationRoot<T,Ranges...>,OpClass>
+                            (mOrigDataPtr,*this,in)))
     {
         static_assert( OpClass::SIZE == decltype(in.rootSteps())::SIZE, "Ext Size mismatch" );
-        return i->pifor(1,in.loop(AssignmentExpr2<T,ParallelOperationRoot<T,Ranges...>,OpClass>
-				  (mOrigDataPtr,*this,in)));
+        return in.loop(AssignmentExpr<T,IOp,ParallelOperationRoot<T,Ranges...>,OpClass>
+                       (mOrigDataPtr,*this,in));
+    }
+    
+    template <typename T, class... Ranges>
+    template <class IOp, class OpClass, class Index>
+    auto ParallelOperationRoot<T,Ranges...>::asx(const OpClass& in, const std::shared_ptr<Index>& i) const
+        -> decltype(i->pifor(1,in.loop(AssignmentExpr<T,IOp,ParallelOperationRoot<T,Ranges...>,OpClass>
+                                      (mOrigDataPtr,*this,in))))
+    {
+        static_assert( OpClass::SIZE == decltype(in.rootSteps())::SIZE, "Ext Size mismatch" );
+        return i->pifor(1,in.loop(AssignmentExpr<T,IOp,ParallelOperationRoot<T,Ranges...>,OpClass>
+                                 (mOrigDataPtr,*this,in)));
     }
 
     template <typename T, class... Ranges>
     template <class OpClass>
-    auto ParallelOperationRoot<T,Ranges...>::plus(const OpClass& in)
-        -> decltype(mIndex.pifor(1,in.loop(AddExpr<T,ParallelOperationRoot<T,Ranges...>,OpClass,OpIndexAff::TARGET>
-                                           (mOrigDataPtr,*this,in))))
+    auto ParallelOperationRoot<T,Ranges...>::assign(const OpClass& in) const
+        -> decltype(this->template asx<IAssign<T>>(in))
     {
-        return mIndex.pifor(1,in.loop(AddExpr<T,ParallelOperationRoot<T,Ranges...>,OpClass,OpIndexAff::TARGET>
-                                      (mOrigDataPtr,*this,in)));
+        return this->template asx<IAssign<T>>(in);
+    }
+
+    template <typename T, class... Ranges>
+    template <class OpClass>
+    auto ParallelOperationRoot<T,Ranges...>::assignExpr(const OpClass& in) const
+        -> decltype(this->template asxExpr<IAssign<T>>(in))
+    {
+        return this->template asxExpr<IAssign<T>>(in);
+    }
+    
+    template <typename T, class... Ranges>
+    template <class OpClass, class Index>
+    auto ParallelOperationRoot<T,Ranges...>::assign(const OpClass& in, const std::shared_ptr<Index>& i) const
+        -> decltype(this->template asx<IAssign<T>>(in,i))
+    {
+        return this->template asx<IAssign<T>>(in,i);
+    }
+
+    template <typename T, class... Ranges>
+    template <class OpClass>
+    auto ParallelOperationRoot<T,Ranges...>::plus(const OpClass& in) const
+        -> decltype(this->template asx<IPlus<T>>(in))
+    {
+        return this->template asx<IPlus<T>>(in);
     }
 
     template <typename T, class... Ranges>
     template <class OpClass, class Index>
     auto ParallelOperationRoot<T,Ranges...>::plus(const OpClass& in, const std::shared_ptr<Index>& i) const
-        -> decltype(i->pifor(1,in.loop(AddExpr<T,ParallelOperationRoot<T,Ranges...>,OpClass>
-				       (mOrigDataPtr,*this,in))))
+        -> decltype(this->template asx<IPlus<T>>(in,i))
     {
-        static_assert( OpClass::SIZE == decltype(in.rootSteps())::SIZE, "Ext Size mismatch" );
-        return i->pifor(1,in.loop(AddExpr<T,ParallelOperationRoot<T,Ranges...>,OpClass>
-				  (mOrigDataPtr,*this,in)));
+        return this->template asx<IPlus<T>>(in,i);
     }
 
     template <typename T, class... Ranges>
diff --git a/src/include/multi_array_operation.h b/src/include/multi_array_operation.h
index bc35d0e..dfbfcde 100644
--- a/src/include/multi_array_operation.h
+++ b/src/include/multi_array_operation.h
@@ -251,10 +251,14 @@ namespace MultiArrayTools
 	template <typename T, typename Op, class ExtType>
 	static inline void f(T*& t, size_t pos, const Op& op, ExtType e)
 	{
+	    VCHECK(pos);
 	    VFunc<F>::selfApply(reinterpret_cast<V*>(t)[pos],op.template vget<V>(e));
 	}
     };
 
+    template <typename T>
+    using xxxplus = plus<T>;
+    
     template <typename T>
     using IAssign = IAccess<identity<T>>;
 
@@ -684,25 +688,39 @@ namespace MultiArrayTools
 
 	ParallelOperationRoot(T* data, const IndexType& ind);
 
-        template <class OpClass>
-        auto assign(const OpClass& in)
-            -> decltype(mIndex.pifor(1,in.loop(AssignmentExpr2<T,ParallelOperationRoot<T,Ranges...>,OpClass,OpIndexAff::TARGET>
-                                               (mOrigDataPtr,*this,in))));
+        template <class IOp, class OpClass>
+        auto asx(const OpClass& in) const
+            -> decltype(mIndex.pifor(1,in.loop(AssignmentExpr<T,IOp,ParallelOperationRoot<T,Ranges...>,OpClass,OpIndexAff::TARGET>
+                                              (mOrigDataPtr,*this,in))));
 
-        template <class OpClass, class Index>
-        auto assign(const OpClass& in, const std::shared_ptr<Index>& i) const
-            -> decltype(i->pifor(1,in.loop(AssignmentExpr2<T,ParallelOperationRoot<T,Ranges...>,OpClass>
+        template <class IOp, class OpClass>
+        auto asxExpr(const OpClass& in) const
+            -> decltype(in.loop(AssignmentExpr<T,IOp,ParallelOperationRoot<T,Ranges...>,OpClass>(mOrigDataPtr,*this,in)));
+            
+        template <class IOp, class OpClass, class Index>
+        auto asx(const OpClass& in, const std::shared_ptr<Index>& i) const
+            -> decltype(i->pifor(1,in.loop(AssignmentExpr<T,IOp,ParallelOperationRoot<T,Ranges...>,OpClass>
                                           (mOrigDataPtr,*this,in))));
+            
+        template <class OpClass>
+        auto assign(const OpClass& in) const
+	    -> decltype(this->template asx<IAssign<T>>(in));
 
 	template <class OpClass>
-        auto plus(const OpClass& in)
-            -> decltype(mIndex.pifor(1,in.loop(AddExpr<T,ParallelOperationRoot<T,Ranges...>,OpClass,OpIndexAff::TARGET>
-                                               (mOrigDataPtr,*this,in))));
+        auto assignExpr(const OpClass& in) const
+	    -> decltype(this->template asxExpr<IAssign<T>>(in));
+	    
+        template <class OpClass, class Index>
+        auto assign(const OpClass& in, const std::shared_ptr<Index>& i) const
+	    -> decltype(this->template asx<IAssign<T>>(in,i));
+	
+	template <class OpClass>
+        auto plus(const OpClass& in) const
+	    -> decltype(this->template asx<IPlus<T>>(in));
 
         template <class OpClass, class Index>
         auto plus(const OpClass& in, const std::shared_ptr<Index>& i) const
-            -> decltype(i->pifor(1,in.loop(AddExpr<T,ParallelOperationRoot<T,Ranges...>,OpClass>
-					   (mOrigDataPtr,*this,in))));
+	    -> decltype(this->template asx<IPlus<T>>(in,i));
 
         template <class OpClass>
         ParallelOperationRoot& operator=(const OpClass& in);
diff --git a/src/include/xfor/xfor.h b/src/include/xfor/xfor.h
index 1ee58f5..4ba9041 100644
--- a/src/include/xfor/xfor.h
+++ b/src/include/xfor/xfor.h
@@ -396,7 +396,9 @@ namespace MultiArrayHelper
 	virtual std::intptr_t vec(size_t vs) override final
 	{
 	    if(mStep == 1 and mMax % vs == 0){
+		VCHECK(vs);
 		mMax /= vs;
+		VCHECK(mMax);
 		return reinterpret_cast<std::intptr_t>(mIndPtr);
 	    }
 	    return mExpr.vec(vs);
@@ -450,10 +452,14 @@ namespace MultiArrayHelper
 	PFor(const IndexClass* indPtr,
 	    size_t step, Expr expr);
 
-	virtual std::intptr_t vec(size_t vs) override final
+	template <size_t VS>
+	auto vec() const
 	{
+	    // statically distinguish!!!
 	    if(mStep == 1 and mMax % vs == 0){
+		VCHECK(vs);
 		mMax /= vs;
+		VCHECK(mMax);
 		return reinterpret_cast<std::intptr_t>(mIndPtr);
 	    }
 	    return mExpr.vec(vs);