diff --git a/src/include/ranges/xfor/pos_type.cc.h b/src/include/ranges/xfor/pos_type.cc.h
index ff36b24..a3cf4d3 100644
--- a/src/include/ranges/xfor/pos_type.cc.h
+++ b/src/include/ranges/xfor/pos_type.cc.h
@@ -238,6 +238,8 @@ namespace CNORXZ
     {
 	static_assert(is_scalar_pos_type<BPosT>::value,
 		      "MPos has to be derived from scalar pos type");
+	static_assert(pos_depth<NPosT>::value < MAX_VMPOS_DEPTH,
+		      "preliminary...");
     }
 
     template <class BPosT, class NPosT>
@@ -246,6 +248,8 @@ namespace CNORXZ
     {
 	static_assert(is_scalar_pos_type<BPosT>::value,
 		      "MPos has to be derived from scalar pos type");
+	static_assert(pos_depth<NPosT>::value < MAX_VMPOS_DEPTH,
+		      "preliminary...");
     }
 
     template <class BPosT, class NPosT>
@@ -255,6 +259,8 @@ namespace CNORXZ
     {
 	static_assert(is_scalar_pos_type<BPosT>::value,
 		      "MPos has to be derived from scalar pos type");
+	static_assert(pos_depth<NPosT>::value < MAX_VMPOS_DEPTH,
+		      "preliminary...");
     }
 
     template <class BPosT, class NPosT>
@@ -264,6 +270,8 @@ namespace CNORXZ
     {
 	static_assert(is_scalar_pos_type<BPosT>::value,
 		      "MPos has to be derived from scalar pos type");
+	static_assert(pos_depth<NPosT>::value < MAX_VMPOS_DEPTH,
+		      "preliminary...");
     }
 
     template <class BPosT, class NPosT>
@@ -309,7 +317,8 @@ namespace CNORXZ
     template <class PosT>
     constexpr auto MPos<BPosT,NPosT>::extend(const PosT& a) const
     {
-	return mNext.extend(a);
+	typedef decltype(mNext.extend(a)) ONPosT;
+	return MPos<BPosT,ONPosT>(*this, mNext.extend(a));
     }
 
     /************
diff --git a/src/include/ranges/xfor/pos_type.h b/src/include/ranges/xfor/pos_type.h
index 041eff9..e9b8b39 100644
--- a/src/include/ranges/xfor/pos_type.h
+++ b/src/include/ranges/xfor/pos_type.h
@@ -16,6 +16,18 @@ namespace CNORXZ
     template <class T>
     struct is_static_pos_type { CXZ_CVAL_FALSE; };
 
+    template <class PosT>
+    struct pos_depth
+    {
+	pos_depth()
+	{
+	    static_assert(is_pos_type<PosT>::value,
+			  "pos_depth is only defined for pos types");
+	}
+
+	static constexpr SizeT value = 1;
+    };
+    
     template <SizeT N>
     class SPos
     {
@@ -237,6 +249,12 @@ namespace CNORXZ
     template <class BPosT, class NPosT> struct is_pos_type<MPos<BPosT,NPosT>> { CXZ_CVAL_TRUE; };
     template <> struct is_pos_type<DPos> { CXZ_CVAL_TRUE; };
     template <> struct is_pos_type<DPosRef> { CXZ_CVAL_TRUE; };
+
+    template <class BPosT, class NPosT>
+    struct pos_depth<MPos<BPosT,NPosT>>
+    {
+	static constexpr SizeT value = pos_depth<NPosT>::value + 1;
+    };
     
 } // end namespace CNORXZInternal
 
diff --git a/src/include/ranges/xfor/vpos_type.cc.h b/src/include/ranges/xfor/vpos_type.cc.h
index 5332973..e4216a8 100644
--- a/src/include/ranges/xfor/vpos_type.cc.h
+++ b/src/include/ranges/xfor/vpos_type.cc.h
@@ -156,7 +156,7 @@ namespace CNORXZ
     template <class PosT1, class PosT2>
     VPos<MPos<PosT1,PosT2>>::VPos(const MPos<PosT1,PosT2>& a) :
 	MPos<PosT1,PosT2>(a),
-	mTRef(this),
+	mTRef(static_cast<const PosT1*>(this)),
 	mNRef(&this->next())
     {}
 	
@@ -225,15 +225,27 @@ namespace CNORXZ
     template <class PosT1, class PosT2>
     Uptr<VPosBase> VPos<MPos<PosT1,PosT2>>::vextend(const UPos& a) const 
     {
-	typedef decltype(this->extend(a)) OPosT;
-	return std::make_unique<VPos<OPosT>>(this->extend(a));
+	if constexpr(pos_depth<PosT2>::value < MAX_VMPOS_DEPTH-1){
+	    typedef decltype(this->extend(a)) OPosT;
+	    return std::make_unique<VPos<OPosT>>(this->extend(a));
+	}
+	else {
+	    CXZ_ERROR("preliminary...");
+	    return nullptr;
+	}
     }
     
     template <class PosT1, class PosT2>
     Uptr<VPosBase> VPos<MPos<PosT1,PosT2>>::vextend(const FPos& a) const 
     {
-	typedef decltype(this->extend(a)) OPosT;
-	return std::make_unique<VPos<OPosT>>(this->extend(a));
+	if constexpr(pos_depth<PosT2>::value < MAX_VMPOS_DEPTH-1){
+	    typedef decltype(this->extend(a)) OPosT;
+	    return std::make_unique<VPos<OPosT>>(this->extend(a));
+	}
+	else {
+	    CXZ_ERROR("preliminary...");
+	    return nullptr;
+	}
     }
     
     /***************
@@ -327,7 +339,9 @@ namespace CNORXZ
 
     template <class PosT1, class PosT2>
     VPosRef<MPos<PosT1,PosT2>>::VPosRef(const MPos<PosT1,PosT2>* c) :
-	mC(c), mTRef(this), mNRef(c->vnext())
+	mC(c),
+	mTRef(static_cast<const PosT1*>(mC)),
+	mNRef(&c->next())
     {}
 
     template <class PosT1, class PosT2>
@@ -368,7 +382,7 @@ namespace CNORXZ
     template <class PosT1, class PosT2>
     const VPosBase* VPosRef<MPos<PosT1,PosT2>>::vnext() const
     {
-	return mNRef;
+	return &mNRef;
     }
 
     template <class PosT1, class PosT2>
@@ -395,15 +409,27 @@ namespace CNORXZ
     template <class PosT1, class PosT2>
     Uptr<VPosBase> VPosRef<MPos<PosT1,PosT2>>::vextend(const UPos& a) const
     {
-	typedef decltype(mC->extend(a)) OPosT;
-	return std::make_unique<VPos<OPosT>>(mC->extend(a));
+	if constexpr(pos_depth<PosT2>::value < MAX_VMPOS_DEPTH-1){
+	    typedef decltype(mC->extend(a)) OPosT;
+	    return std::make_unique<VPos<OPosT>>(mC->extend(a));
+	}
+	else {
+	    CXZ_ERROR("preliminary...");
+	    return nullptr;
+	}
     }
 
     template <class PosT1, class PosT2>
     Uptr<VPosBase> VPosRef<MPos<PosT1,PosT2>>::vextend(const FPos& a) const
     {
-	typedef decltype(mC->extend(a)) OPosT;
-	return std::make_unique<VPos<OPosT>>(mC->extend(a));
+	if constexpr(pos_depth<PosT2>::value < MAX_VMPOS_DEPTH-1){
+	    typedef decltype(mC->extend(a)) OPosT;
+	    return std::make_unique<VPos<OPosT>>(mC->extend(a));
+	}
+	else {
+	    CXZ_ERROR("preliminary...");
+	    return nullptr;
+	}
     }
 }
 
diff --git a/src/include/ranges/xfor/vpos_type.h b/src/include/ranges/xfor/vpos_type.h
index 1cf36cf..c217ba0 100644
--- a/src/include/ranges/xfor/vpos_type.h
+++ b/src/include/ranges/xfor/vpos_type.h
@@ -4,6 +4,8 @@
 
 #include "base/base.h"
 
+#define MAX_VMPOS_DEPTH 10
+
 namespace CNORXZ
 {
     class VPosBase
@@ -126,8 +128,6 @@ 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 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;
     };
diff --git a/src/tests/xfor_unit_test.cc b/src/tests/xfor_unit_test.cc
index fa45cf3..6095c8a 100644
--- a/src/tests/xfor_unit_test.cc
+++ b/src/tests/xfor_unit_test.cc
@@ -97,6 +97,25 @@ namespace
 	EXPECT_EQ(mp5.val(), mUp2.val() * mUp1.val());
 	EXPECT_EQ(mp5.next().val(), mS4p.val() * mUp1.val());
     }
+
+    TEST_F(Pos_Test, Extend)
+    {
+	auto mp1 = mkMPos(mS2p, mUp1);
+	auto mp2 = mkMPos(mUp2, mS4p);
+	auto mp3 = mp1.extend(mUp2);
+	auto mp4 = mp2.extend(mS2p);
+
+	EXPECT_EQ( mp3.size(), 3u );
+	EXPECT_EQ( mp4.size(), 3u );
+
+	EXPECT_EQ( mp3.val(), mS2p.val() );
+	EXPECT_EQ( mp3.next().val(), mUp1.val() );
+	EXPECT_EQ( mp3.next().next().val(), mUp2.val() );
+
+    	EXPECT_EQ( mp4.val(), mUp2.val() );
+	EXPECT_EQ( mp4.next().val(), mS4p.val() );
+	EXPECT_EQ( mp4.next().next().val(), mS2p.val() );
+    }
     
     TEST_F(Pos_Test, Dyn)
     {