diff --git a/src/include/allocator.h b/src/include/allocator.h
index 2cfdd2c..aaff744 100644
--- a/src/include/allocator.h
+++ b/src/include/allocator.h
@@ -68,7 +68,7 @@ namespace MultiArrayTools
 {
     template <typename T>
     using vector = std::vector<T,MultiArrayHelper::Allocator<T>>;
-
+    
 } // namespace MultiArrayTools
 
 #endif
diff --git a/src/include/map_range.h b/src/include/map_range.h
index 2bc9642..a67f350 100644
--- a/src/include/map_range.h
+++ b/src/include/map_range.h
@@ -38,10 +38,11 @@ namespace MultiArrayTools
     class OpExpr
     {
     public:
+	typedef decltype(mkMapOp(std::declval<MapF>(), std::declval<IndexPack>())) Op;
 	typedef SingleIndex<typename MapF::value_type,STYPE> OIType;
 	//typedef typename MapF::IndexPack IndexPack;
 	static constexpr size_t LAYER = Expr::LAYER + 1;
-	static constexpr size_t SIZE = Expr::SIZE;
+	static constexpr size_t SIZE = Expr::SIZE + Op::SIZE;
 	
     private:
 	OpExpr() = default;
@@ -51,8 +52,6 @@ namespace MultiArrayTools
 	size_t mMax;
 	size_t mStep;
 	Expr mExpr;
-
-	typedef decltype(mkMapOp(std::declval<MapF>(), std::declval<IndexPack>())) Op;
 	Op mOp;
 	
 	typedef decltype(mOp.rootSteps(std::declval<intptr_t>()).extend( mExpr.rootSteps(std::declval<intptr_t>()) )) ExtType;
diff --git a/src/include/multi_array_operation.cc.h b/src/include/multi_array_operation.cc.h
index e69f66e..cee7635 100644
--- a/src/include/multi_array_operation.cc.h
+++ b/src/include/multi_array_operation.cc.h
@@ -346,6 +346,7 @@ namespace MultiArrayTools
     auto OperationRoot<T,Ranges...>::assign(const OpClass& in)
         -> decltype(mIndex.ifor(1,in.loop(AssignmentExpr<T,OpClass>(mOrigDataPtr,in))))
     {
+        static_assert( OpClass::SIZE == decltype(in.rootSteps())::SIZE, "Ext Size mismatch" );
         return mIndex.ifor(1,in.loop(AssignmentExpr<T,OpClass>(mOrigDataPtr,in)));
     }
 
@@ -354,6 +355,7 @@ namespace MultiArrayTools
     auto OperationRoot<T,Ranges...>::plus(const OpClass& in)
         -> decltype(mIndex.ifor(1,in.loop(AddExpr<T,OpClass>(mOrigDataPtr,in))))
     {
+        static_assert( OpClass::SIZE == decltype(in.rootSteps())::SIZE, "Ext Size mismatch" );
         return mIndex.ifor(1,in.loop(AddExpr<T,OpClass>(mOrigDataPtr,in)));
     }
 
@@ -556,9 +558,9 @@ namespace MultiArrayTools
     }
 
     template <typename T>
-    MExt<void> OperationValue<T>::rootSteps(std::intptr_t iPtrNum) const
+    None OperationValue<T>::rootSteps(std::intptr_t iPtrNum) const
     {
-	return MExt<void>(0);
+	return None();
     }
 
     template <typename T>
diff --git a/src/include/multi_array_operation.h b/src/include/multi_array_operation.h
index 0cd01ad..9691b82 100644
--- a/src/include/multi_array_operation.h
+++ b/src/include/multi_array_operation.h
@@ -388,7 +388,7 @@ namespace MultiArrayTools
 	typedef ContainerRange<T,NullRange> CRange;
 	typedef ContainerIndex<T,NullIndex> IndexType;
 
-	static constexpr size_t SIZE = 1;
+	static constexpr size_t SIZE = 0;
         static constexpr bool CONT = true;
 
 	OperationValue(const T& val);
@@ -399,7 +399,7 @@ namespace MultiArrayTools
 	template <class ET>
 	inline OperationValue& set(ET pos);
 
-	MExt<void> rootSteps(std::intptr_t iPtrNum = 0) const; // nullptr for simple usage with decltype
+	None rootSteps(std::intptr_t iPtrNum = 0) const; // nullptr for simple usage with decltype
 
 	template <class Expr>
 	Expr loop(Expr exp) const;
diff --git a/src/include/pack_num.h b/src/include/pack_num.h
index d369b17..590246c 100644
--- a/src/include/pack_num.h
+++ b/src/include/pack_num.h
@@ -74,7 +74,7 @@ namespace MultiArrayHelper
 	static inline T mkOpExpr(std::shared_ptr<OpFunction> f, const ETuple& pos, const OpTuple& ops, Args... args)
 	{
 	    typedef typename std::remove_reference<decltype(std::get<N>(ops))>::type NextOpType;
-	    static_assert(LAST > NextOpType::SIZE, "inconsistent array positions");
+	    static_assert(LAST >= NextOpType::SIZE, "inconsistent array positions");
 	    static constexpr size_t NEXT = LAST - NextOpType::SIZE;
 	    typedef decltype(std::get<N>(ops).get(Getter<NEXT>::template getX<ETuple>( pos ))) ArgT;
 	    return PackNum<N-1>::template mkOpExpr<NEXT,T,ETuple,OpTuple,OpFunction,ArgT,Args...>
diff --git a/src/include/xfor/exttype.h b/src/include/xfor/exttype.h
index 918ed6c..31dbcdd 100644
--- a/src/include/xfor/exttype.h
+++ b/src/include/xfor/exttype.h
@@ -52,7 +52,8 @@ namespace MultiArrayHelper
 	
     public:
 	
-	static constexpr size_t NUM = X::NUM + 1;
+	static constexpr size_t NUM = X::SIZE;
+        static constexpr size_t SIZE = NUM + 1;
 
 	MExt() = default;
 	MExt(const MExt& in) = default;
@@ -71,6 +72,9 @@ namespace MultiArrayHelper
 	template <size_t N>
 	inline MExt(const std::array<size_t,N>& arr);
 
+        template <class Y>
+        inline MExt(const MExt<Y>& y);
+        
 	inline const size_t& val() const;
 	inline const X& next() const;
 
@@ -88,6 +92,27 @@ namespace MultiArrayHelper
 	
     };    
 
+    struct None
+    {
+        None() = default;
+        None(const None& in) = default;
+        None(None&& in) = default;
+        None& operator=(const None& in) = default;
+        None& operator=(None&& in) = default;
+        
+        template <class Y>
+        None(const Y& y) {}
+        
+        static constexpr size_t SIZE = 0;
+        
+        inline None operator+(const None& in) const { return None(); }
+	inline None operator*(size_t in) const { return None(); }
+        
+        template <class Y>
+        Y extend(const Y& y) const
+        { return y; }
+    };
+    
     template <>
     class MExt<void>
     {
@@ -98,7 +123,8 @@ namespace MultiArrayHelper
     public:
 	
 	static constexpr size_t NUM = 0;
-
+        static constexpr size_t SIZE = NUM + 1;
+        
 	MExt() = default;
 	MExt(const MExt& in) = default;
 	MExt& operator=(const MExt& in) = default;
@@ -107,17 +133,24 @@ namespace MultiArrayHelper
 	
 	inline MExt(size_t ext);
 
+        inline MExt(size_t y, const None& z) : mExt(y) {}
+
 	template <class Z>
 	inline MExt(size_t y, const Z& z);
-	
+
+
+
 	template <class Y, class Z>
 	inline MExt(const Y& y, const Z& z);
 
 	template <size_t N>
 	inline MExt(const std::array<size_t,N>& arr);
 
+        template <class Y>
+        inline MExt(const MExt<Y>& y);
+        
 	inline const size_t& val() const;
-	inline size_t next() const { return 0; }
+	inline None next() const { return None(); }
 
         template <size_t N>
         inline auto nn() const
@@ -161,7 +194,12 @@ namespace MultiArrayHelper
     template <class Y, class Z>
     inline MExt<X>::MExt(const Y& y, const Z& z) :
 	mExt(y.val()), mNext(y.next(), z) {}
-    
+
+    template <class X>
+    template <class Y>
+    inline MExt<X>::MExt(const MExt<Y>& y) :
+        mExt(y.val()), mNext(y.next()) {}
+
     template <class X>
     inline const size_t& MExt<X>::val() const
     {
@@ -207,6 +245,10 @@ namespace MultiArrayHelper
     inline MExt<void>::MExt(const std::array<size_t,N>& arr) :
 	mExt(std::get<NUM>(arr)) {}
 
+    template <class Y>
+    inline MExt<void>::MExt(const MExt<Y>& y) :
+        mExt(y.val()) {}
+
     //template <>
     inline const size_t& MExt<void>::val() const
     {