diff --git a/src/include/operation/op_utility.cc.h b/src/include/operation/op_utility.cc.h
new file mode 100644
index 0000000..2b5da71
--- /dev/null
+++ b/src/include/operation/op_utility.cc.h
@@ -0,0 +1,54 @@
+
+#ifndef __cxz_op_utility_cc_h__
+#define __cxz_op_utility_cc_h__
+
+#include "op_utility.h"
+#include "xpr/pos_type.h"
+#include "operation/op_type.h"
+
+namespace CNORXZ
+{
+    template <SizeT I, class PosT>
+    constexpr decltype(auto) pos_get(const PosT& pos)
+    {
+	static_assert(I < static_pos_size<PosT>::value, "index out of range");
+	if constexpr(I == 0){
+	    return pos;
+	}
+	else {
+	    return pos_get<I-1>(pos.next());
+	}
+    }
+
+    template <SizeT J, SizeT I, SizeT... Is>
+    constexpr SizeT sum_index_sequence(std::index_sequence<I,Is...> is)
+    {
+	static_assert(J < sizeof...(Is)+1, "index out of range");
+	if constexpr(J == 0){
+	    return 0;
+	}
+	else {
+	    return sum_index_sequence<J-1>(std::index_sequence<Is...>{}) + I;
+	}
+    }
+
+    template <class F, class PosT, class OpTuple, class OpSizes, SizeT... Is>
+    inline void pos_unpack_args_i(const F& f, const PosT& pos, const OpTuple& args,
+				  OpSizes opsizes, std::index_sequence<Is...> is)
+    {
+	f(std::get<Is>(args).get(pos_get<sum_index_sequence<Is>(opsizes)>(pos))...);
+    }
+
+    template <class F, class PosT, class... Ops>
+    inline void pos_unpack_args(const F& f, const PosT& pos, const Tuple<Ops...>& args)
+    {
+	static_assert(is_pos_type<PosT>::value, "got non-pos-type");
+	static_assert((is_operation<Ops>::value and ...), "got non-operation type");
+	typedef std::make_index_sequence<sizeof...Ops> Idxs;
+	typedef std::index_sequence<op_size<Ops>::value...> OpSizes;
+	pos_unpack_args_i(f, pos, args, OpSizes{}, Idx{});
+    }
+
+}
+
+#endif
diff --git a/src/include/operation/op_utility.h b/src/include/operation/op_utility.h
new file mode 100644
index 0000000..d04d9d2
--- /dev/null
+++ b/src/include/operation/op_utility.h
@@ -0,0 +1,25 @@
+
+#ifndef __cxz_op_utility_h__
+#define __cxz_op_utility_h__
+
+#include "base/base.h"
+
+namespace CNORXZ
+{
+    template <SizeT I, class PosT>
+    constexpr decltype(auto) pos_get(const PosT& pos);
+    
+    template <SizeT J, SizeT... Is>
+    constexpr SizeT sum_index_sequence(std::index_sequence<Is...> is);
+
+    template <class F, class PosT, class OpTuple, class OpSizes, SizeT... Is>
+    inline void pos_unpack_args_i(const F& f, const PosT& pos, const OpTuple& args,
+				  OpSizes opsizes, std::index_sequence<Is...> is);
+    
+    template <class F, class PosT, class... Ops>
+    inline void pos_unpack_args(const F& f, const PosT& pos, const Tuple<Ops...>& args);
+    
+    
+}
+
+#endif
diff --git a/src/include/operation/op_xpr.cc.h b/src/include/operation/op_xpr.cc.h
index 21231e8..8488766 100644
--- a/src/include/operation/op_xpr.cc.h
+++ b/src/include/operation/op_xpr.cc.h
@@ -3,6 +3,7 @@
 #define __cxz_op_expr_cc_h__
 
 #include "op_expr.h"
+#include "op_utility.h"
 
 namespace CNORXZ
 {
@@ -18,7 +19,7 @@ namespace CNORXZ
     template <class PosT1, class PosT2>
     inline SizeT OpXpr<F,Ops...>::operator()(const PosT& last) const
     {
-	pos_unpack_args(mF,mOps,last); // utility function (to be implemented)
+	pos_unpack_args(mF,last,mOps); // utility function (to be implemented)
 	// depending on whether Ops[N] is static or not call statically or dynamically .next()
 	return 0;
     }
diff --git a/src/include/xpr/pos_type.cc.h b/src/include/xpr/pos_type.cc.h
index 270d750..febcdb5 100644
--- a/src/include/xpr/pos_type.cc.h
+++ b/src/include/xpr/pos_type.cc.h
@@ -373,7 +373,8 @@ namespace CNORXZ
 	return mC->vval();
     }
     
-    inline DPosRef DPos::next() const
+    //inline DPosRef DPos::next() const
+    inline DPosRef DPos::sub() const
     {
 	return DPosRef(mC->vnext());
     }
@@ -416,9 +417,10 @@ namespace CNORXZ
     }
 	
     template <class PosT>
-    inline DPos DPos::extend(const PosT& a) const
+    inline decltype(auto) DPos::extend(const PosT& a) const
     {
-	return DPos(mC->vextend( a ));
+	//return DPos(mC->vextend( a ));
+	return MPos<DPos,PosT>(*this,a);
     }
 
     /***************
@@ -447,7 +449,8 @@ namespace CNORXZ
 	return mP->vval();
     }
     
-    inline DPosRef DPosRef::next() const
+    //inline DPosRef DPosRef::next() const
+    inline DPosRef DPosRef::sub() const
     {
 	return DPosRef(mP->vnext());
     }
@@ -490,9 +493,10 @@ namespace CNORXZ
     }	
 
     template <class PosT>
-    inline DPos DPosRef::extend(const PosT& a) const
+    inline decltype(auto) DPosRef::extend(const PosT& a) const
     {
-	return DPos(mP->vextend( a ));
+	//return DPos(mP->vextend( a ));
+	return MPos<DPos,PosT>(*this,a);
     }
     
     /************
diff --git a/src/include/xpr/pos_type.h b/src/include/xpr/pos_type.h
index d879405..75b5610 100644
--- a/src/include/xpr/pos_type.h
+++ b/src/include/xpr/pos_type.h
@@ -154,6 +154,7 @@ namespace CNORXZ
 	constexpr auto extend(const PosT& a) const;
     };
 
+    // treat as scalar pos!!!
     class DPos : public ObjHandle<VPosBase>
     {
     public:
@@ -172,7 +173,8 @@ namespace CNORXZ
 	inline const VPosBase* vpos() const;
 	inline SizeT size() const;
 	inline SizeT val() const;
-	inline DPosRef next() const;
+	inline DPosRef sub() const;
+	//inline DPosRef next() const;
 
 	template <class PosT>
 	inline DPos operator+(const PosT& a) const;
@@ -185,7 +187,7 @@ namespace CNORXZ
 	inline DPos operator()(const PosT& a) const;
 	
 	template <class PosT>
-	inline DPos extend(const PosT& a) const;
+	inline decltype(auto) extend(const PosT& a) const;
     };
 
     class DPosRef
@@ -202,7 +204,8 @@ namespace CNORXZ
 	inline const VPosBase* vpos() const;
 	inline SizeT size() const;
 	inline SizeT val() const;
-	inline DPosRef next() const;
+	inline DPosRef sub() const;
+	//inline DPosRef next() const;
 
 	template <class PosT>
 	inline DPos operator+(const PosT& a) const;
@@ -215,7 +218,7 @@ namespace CNORXZ
 	inline DPos operator()(const PosT& a) const;
 	
 	template <class PosT>
-	inline DPos extend(const PosT& a) const;
+	inline decltype(auto) extend(const PosT& a) const;
     };
 
     // for common call of extension vector elements
@@ -261,7 +264,7 @@ namespace CNORXZ
     struct is_epos_type { CXZ_CVAL_FALSE; };
     
     template <class PosT>
-    struct pos_depth
+    struct static_pos_size
     {
 	static constexpr SizeT value = is_pos_type<PosT>::value ? 1 : 0;
     };
@@ -322,6 +325,8 @@ namespace CNORXZ
 
     template <> struct is_pos_type<DPos> { CXZ_CVAL_TRUE; };
     template <> struct is_pos_type<DPosRef> { CXZ_CVAL_TRUE; };
+    template <> struct is_scalar_pos_type<DPos> { CXZ_CVAL_TRUE; };
+    template <> struct is_scalar_pos_type<DPosRef> { CXZ_CVAL_TRUE; };
 
     template <class BPosT, class... OPosTs> struct is_pos_type<EPos<BPosT,OPosTs...>> { CXZ_CVAL_TRUE; };
     template <class BPosT, class... OPosTs> struct is_scalar_pos_type<EPos<BPosT,OPosTs...>>
@@ -331,9 +336,9 @@ namespace CNORXZ
     template <class BPosT, class... OPosTs> struct is_epos_type<EPos<BPosT,OPosTs...>> { CXZ_CVAL_TRUE; };
     
     template <class BPosT, class NPosT>
-    struct pos_depth<MPos<BPosT,NPosT>>
+    struct static_pos_size<MPos<BPosT,NPosT>>
     {
-	static constexpr SizeT value = pos_depth<NPosT>::value + 1;
+	static constexpr SizeT value = static_pos_size<NPosT>::value + 1;
     };
 
     template <class BPosT, class... OPosTs>
diff --git a/src/include/xpr/vpos_type.cc.h b/src/include/xpr/vpos_type.cc.h
index cbf1ae4..69f6461 100644
--- a/src/include/xpr/vpos_type.cc.h
+++ b/src/include/xpr/vpos_type.cc.h
@@ -10,7 +10,7 @@ namespace CNORXZ
     /****************
      *   VPosBase   *
      ****************/
-
+    /*
     template <SizeT N>
     inline Uptr<VPosBase> VPosBase::vextend(const SPos<N>& a) const
     {
@@ -28,7 +28,7 @@ namespace CNORXZ
     {
 	return this->vextend(static_cast<const PosT1&>(a))->vextend(a.next());
     }
-
+    */
     /************
      *   VPos   *
      ************/
@@ -88,7 +88,7 @@ namespace CNORXZ
 	typedef decltype((*this)(UPos(a->vval()))) OPosT;
 	return std::make_unique<VPos<OPosT>>( (*this)(UPos(a->vval())) );
     }
-
+    /*
     template <class PosT>
     Uptr<VPosBase> VPos<PosT>::vextend(const UPos& a) const
     {
@@ -116,7 +116,7 @@ namespace CNORXZ
 	typedef decltype(this->extend(DPos(a))) OPosT;
 	return std::make_unique<VPos<OPosT>>(this->extend(DPos(a)));
     }
-
+    */
     /******************
      *   VPos<MPos>   *
      ******************/
@@ -207,7 +207,7 @@ namespace CNORXZ
 	typedef decltype( (*this)(UPos(a->vval())) ) OPosT;
 	return std::make_unique<VPos<OPosT>>( (*this)(UPos(a->vval())) );
     }
-
+    /*
     template <class PosT1, class PosT2>
     Uptr<VPosBase> VPos<MPos<PosT1,PosT2>>::vextend(const UPos& a) const 
     {
@@ -235,7 +235,7 @@ namespace CNORXZ
 	typedef decltype(this->extend(DPos(a))) OPosT;
 	return std::make_unique<VPos<OPosT>>(this->extend(DPos(a)));
     }
-    
+    */
     /***************
      *   VPosRef   *
      ***************/
@@ -295,7 +295,7 @@ namespace CNORXZ
 	typedef decltype( (*mC)(UPos(a->vval())) ) OPosT;
 	return std::make_unique<VPos<OPosT>>( (*mC)(UPos(a->vval())) );
     }
-
+    /*
     template <class PosT>
     Uptr<VPosBase> VPosRef<PosT>::vextend(const UPos& a) const 
     {
@@ -323,7 +323,7 @@ namespace CNORXZ
 	typedef decltype(mC->extend(DPos(a))) OPosT;
 	return std::make_unique<VPos<OPosT>>(mC->extend(DPos(a)));
     }
-
+    */
     /*********************
      *   VPosRef<MPos>   *
      *********************/
@@ -393,7 +393,7 @@ namespace CNORXZ
 	typedef decltype( (*mC)(UPos(a->vval())) ) OPosT;
 	return std::make_unique<VPos<OPosT>>( (*mC)(UPos(a->vval())) );
     }
-
+    /*
     template <class PosT1, class PosT2>
     Uptr<VPosBase> VPosRef<MPos<PosT1,PosT2>>::vextend(const UPos& a) const
     {
@@ -421,7 +421,7 @@ namespace CNORXZ
 	typedef decltype(mC->extend(DPos(a))) OPosT;
 	return std::make_unique<VPos<OPosT>>(mC->extend(DPos(a)));
     }
-    
+    */
 }
 
 #endif
diff --git a/src/include/xpr/vpos_type.h b/src/include/xpr/vpos_type.h
index 0f6b507..f2f85ec 100644
--- a/src/include/xpr/vpos_type.h
+++ b/src/include/xpr/vpos_type.h
@@ -21,11 +21,11 @@ namespace CNORXZ
 	virtual Uptr<VPosBase> vplus(const VPosBase* a) const = 0;
 	virtual Uptr<VPosBase> vtimes(const VPosBase* a) const = 0;
 	virtual Uptr<VPosBase> vexec(const VPosBase* a) const = 0;
-	virtual Uptr<VPosBase> vextend(const UPos& a) const = 0;
-	virtual Uptr<VPosBase> vextend(const FPos& a) const = 0;
-	virtual Uptr<VPosBase> vextend(const DPos& a) const = 0;
-	virtual Uptr<VPosBase> vextend(const DPosRef& a) const = 0;
-
+	//virtual Uptr<VPosBase> vextend(const UPos& a) const = 0;
+	//virtual Uptr<VPosBase> vextend(const FPos& a) const = 0;
+	//virtual Uptr<VPosBase> vextend(const DPos& a) const = 0;
+	//virtual Uptr<VPosBase> vextend(const DPosRef& a) const = 0;
+	/*
 	template <SizeT N>
 	inline Uptr<VPosBase> vextend(const SPos<N>& a) const;
 
@@ -34,7 +34,7 @@ namespace CNORXZ
 
 	template <class PosT1, class PosT2>
 	inline Uptr<VPosBase> vextend(const MPos<PosT1,PosT2>& a) const;
-	
+	*/
     };
 
     template <class PosT>
@@ -53,10 +53,10 @@ namespace CNORXZ
 	virtual Uptr<VPosBase> vplus(const VPosBase* a) const override final;
 	virtual Uptr<VPosBase> vtimes(const VPosBase* a) const override final;
 	virtual Uptr<VPosBase> vexec(const VPosBase* a) const override final;
-	virtual Uptr<VPosBase> vextend(const UPos& a) const override final;
-	virtual Uptr<VPosBase> vextend(const FPos& a) const override final;
-	virtual Uptr<VPosBase> vextend(const DPos& a) const override final;
-	virtual Uptr<VPosBase> vextend(const DPosRef& a) const override final;
+	//virtual Uptr<VPosBase> vextend(const UPos& a) const override final;
+	//virtual Uptr<VPosBase> vextend(const FPos& a) const override final;
+	//virtual Uptr<VPosBase> vextend(const DPos& a) const override final;
+	//virtual Uptr<VPosBase> vextend(const DPosRef& a) const override final;
     };
 
     template <class PosT1, class PosT2>
@@ -81,10 +81,10 @@ namespace CNORXZ
 	virtual Uptr<VPosBase> vplus(const VPosBase* a) const override final;
 	virtual Uptr<VPosBase> vtimes(const VPosBase* a) const override final;
 	virtual Uptr<VPosBase> vexec(const VPosBase* a) const override final;
-	virtual Uptr<VPosBase> vextend(const UPos& a) const override final;
-	virtual Uptr<VPosBase> vextend(const FPos& a) const override final;
-	virtual Uptr<VPosBase> vextend(const DPos& a) const override final;
-	virtual Uptr<VPosBase> vextend(const DPosRef& a) const override final;
+	//virtual Uptr<VPosBase> vextend(const UPos& a) const override final;
+	//virtual Uptr<VPosBase> vextend(const FPos& a) const override final;
+	//virtual Uptr<VPosBase> vextend(const DPos& a) const override final;
+	//virtual Uptr<VPosBase> vextend(const DPosRef& a) const override final;
     };
 
     template <class PosT>
@@ -105,10 +105,10 @@ namespace CNORXZ
 	virtual Uptr<VPosBase> vplus(const VPosBase* a) const override final;
 	virtual Uptr<VPosBase> vtimes(const VPosBase* a) const override final;
 	virtual Uptr<VPosBase> vexec(const VPosBase* a) const override final;
-	virtual Uptr<VPosBase> vextend(const UPos& a) const override final;
-	virtual Uptr<VPosBase> vextend(const FPos& a) const override final;
-	virtual Uptr<VPosBase> vextend(const DPos& a) const override final;
-	virtual Uptr<VPosBase> vextend(const DPosRef& a) const override final;
+	//virtual Uptr<VPosBase> vextend(const UPos& a) const override final;
+	//virtual Uptr<VPosBase> vextend(const FPos& a) const override final;
+	//virtual Uptr<VPosBase> vextend(const DPos& a) const override final;
+	//virtual Uptr<VPosBase> vextend(const DPosRef& a) const override final;
 
     };
 
@@ -133,10 +133,10 @@ namespace CNORXZ
 	virtual Uptr<VPosBase> vplus(const VPosBase* a) const override final;
 	virtual Uptr<VPosBase> vtimes(const VPosBase* a) const override final;
 	virtual Uptr<VPosBase> vexec(const VPosBase* a) const override final;
-	virtual Uptr<VPosBase> vextend(const UPos& a) const override final;
-	virtual Uptr<VPosBase> vextend(const FPos& a) const override final;
-	virtual Uptr<VPosBase> vextend(const DPos& a) const override final;
-	virtual Uptr<VPosBase> vextend(const DPosRef& a) const override final;
+	//virtual Uptr<VPosBase> vextend(const UPos& a) const override final;
+	//virtual Uptr<VPosBase> vextend(const FPos& a) const override final;
+	//virtual Uptr<VPosBase> vextend(const DPos& a) const override final;
+	//virtual Uptr<VPosBase> vextend(const DPosRef& a) const override final;
     };
 
     
diff --git a/src/tests/xpr_unit_test.cc b/src/tests/xpr_unit_test.cc
index 1aba079..30f67b2 100644
--- a/src/tests/xpr_unit_test.cc
+++ b/src/tests/xpr_unit_test.cc
@@ -144,21 +144,21 @@ namespace
 	EXPECT_EQ(dp5.size(), 2u);
 
 	EXPECT_EQ(dp1.val(), mS2p.val());
-	EXPECT_EQ(dp1.next().val(), mUp1.val());
+	EXPECT_EQ(dp1.sub().val(), mUp1.val());
 	EXPECT_EQ(dp2.val(), mUp2.val());
-	EXPECT_EQ(dp2.next().val(), mS4p.val());
+	EXPECT_EQ(dp2.sub().val(), mS4p.val());
 	EXPECT_EQ(dp3a.val(), mS2p.val() + mUp2.val());
-	EXPECT_EQ(dp3a.next().val(), mUp1.val() + mS4p.val());
+	EXPECT_EQ(dp3a.sub().val(), mUp1.val() + mS4p.val());
 	EXPECT_EQ(dp3b.val(), mS2p.val() + mUp2.val());
-	EXPECT_EQ(dp3b.next().val(), mUp1.val() + mS4p.val());
+	EXPECT_EQ(dp3b.sub().val(), mUp1.val() + mS4p.val());
 	EXPECT_EQ(dp4.val(), mS2p.val() * mS2p.val());
-	EXPECT_EQ(dp4.next().val(), mUp1.val() * mS2p.val());
+	EXPECT_EQ(dp4.sub().val(), mUp1.val() * mS2p.val());
 	EXPECT_EQ(dp5.val(), mUp2.val() * mUp1.val());
-	EXPECT_EQ(dp5.next().val(), mS4p.val() * mUp1.val());
+	EXPECT_EQ(dp5.sub().val(), mS4p.val() * mUp1.val());
     }
 
     TEST_F(Pos_Test, DynExtend)
-    {
+    {/*
 	DPos dp1(mkMPos(static_cast<UPos>(mS2p), mUp1));
 	DPos dp2(mkMPos(mUp2, static_cast<UPos>(mS4p)));
 	auto dp3 = dp1.extend(mUp2);
@@ -188,7 +188,7 @@ namespace
 	EXPECT_EQ( dp6.val(), dp3.val() * dp01.val() );
 	EXPECT_EQ( dp6.next().val(), dp3.next().val() * dp01.val() );
 	EXPECT_EQ( dp6.next().next().val(), dp3.next().next().val() * dp01.val() );
-    }
+     */}
 }
 
 int main(int argc, char** argv)