diff --git a/src/include/ranges/index_base.h b/src/include/ranges/index_base.h
index 23be09c..335ee85 100644
--- a/src/include/ranges/index_base.h
+++ b/src/include/ranges/index_base.h
@@ -36,19 +36,19 @@ namespace CNORXZ
 	bool operator>(const IndexInterface& in) const;
 	bool operator<=(const IndexInterface& in) const;
 	bool operator>=(const IndexInterface& in) const;
-	auto operator*() const { return THIS().operator*(); }
-	auto operator->() const { return THIS().operator->(); }
+	decltype(auto) operator*() const { return THIS().operator*(); }
+	decltype(auto) operator->() const { return THIS().operator->(); }
 
 	Int pp(PtrId idxPtrNum) { return THIS().pp(idxPtrNum); }
 	Int mm(PtrId idxPtrNum) { return THIS().mm(idxPtrNum); }
 	
 	SizeT dim() const { return THIS().dim(); }
-	auto range() const { return THIS().range(); }
+	decltype(auto) range() const { return THIS().range(); }
 	SizeT getStepSize(PtrId iptr) const { return THIS().getStepSize(iptr); }
 	//SizeT getStepSize(SizeT n) const { return THIS().getStepSize(n); }
 	
 	String stringMeta() const { return THIS().stringMeta(); }
-	auto meta() const { return THIS().meta(); }
+	decltype(auto) meta() const { return THIS().meta(); }
 	I& at(const MetaType& meta) { return THIS().at(meta); }
 	/*
 	template <class Expr>
diff --git a/src/include/ranges/xpr/for.cc.h b/src/include/ranges/xpr/for.cc.h
index c924d41..b91817a 100644
--- a/src/include/ranges/xpr/for.cc.h
+++ b/src/include/ranges/xpr/for.cc.h
@@ -12,18 +12,18 @@ namespace CNORXZ
      *   For   *
      ***********/
     
-    template <class Xpr>
-    constexpr For<Xpr>::For(SizeT size, PtrId iptrId, SizeT step, const Xpr& xpr) :
+    template <SizeT L, class Xpr>
+    constexpr For<L,Xpr>::For(SizeT size, const IndexId<L>& id, SizeT step, const Xpr& xpr) :
 	mSize(size),
-	mIptrId(iptrId),
+	mId(id),
 	mStep(step),
 	mXpr(xpr),
-	mExt(mXpr.rootSteps(mIptrId))
+	mExt(mXpr.rootSteps(mId))
     {}
 
-    template <class Xpr>
+    template <SizeT L, class Xpr>
     template <class PosT1, class PosT2>
-    inline SizeT For<Xpr>::operator()(const PosT1& mlast, const PosT2& last) const
+    inline SizeT For<L,Xpr>::operator()(const PosT1& mlast, const PosT2& last) const
     {
 	for(SizeT i = 0; i != mSize; ++i){
 	    const auto mpos = mlast + mStep * UPos(i);
@@ -33,8 +33,8 @@ namespace CNORXZ
 	return 0;
     }
 	
-    template <class Xpr>
-    inline SizeT For<Xpr>::operator()() const
+    template <SizeT L, class Xpr>
+    inline SizeT For<L,Xpr>::operator()() const
     {
 	for(SizeT i = 0; i != mSize; ++i){
 	    const SizeT mpos = mStep * UPos(i);
@@ -44,17 +44,11 @@ namespace CNORXZ
 	return 0;
     }
 
-    template <class Xpr>
-    inline auto For<Xpr>::rootSteps(PtrId ptrId) const
+    template <SizeT L, class Xpr>
+    template <SizeT I>
+    inline decltype(auto) For<L,Xpr>::rootSteps(const IndexId<I>& id) const
     {
-	return mXpr.rootSteps(ptrId);
-    }
-
-    template <class Xpr>
-    template <SizeT L>
-    constexpr auto For<Xpr>::staticRootSteps(PtrId ptrId) const
-    {
-	return mXpr.template staticRootSteps<L>(ptrId);
+	return mXpr.rootSteps(id);
     }
 
 
@@ -63,10 +57,10 @@ namespace CNORXZ
      *************/
 
     template <SizeT N, SizeT L, SizeT S, class Xpr>
-    constexpr SLFor<N,L,S,Xpr>::SLFor(PtrId iptrId, const Xpr& xpr) :
-	mIptrId(iptrId),
+    constexpr SLFor<N,L,S,Xpr>::SLFor(const IndexId<L>& id, const Xpr& xpr) :
+	mId(id),
 	mXpr(xpr),
-	mExt(mXpr.template staticRootSteps<L>(mIptrId))
+	mExt(mXpr.RootSteps(mId))
     {}
 
     template <SizeT N, SizeT L, SizeT S, class Xpr>
@@ -83,16 +77,10 @@ namespace CNORXZ
     }
 
     template <SizeT N, SizeT L, SizeT S, class Xpr>
-    constexpr auto SLFor<N,L,S,Xpr>::rootSteps(PtrId ptrId) const
+    template <SizeT I>
+    constexpr decltype(auto) SLFor<N,L,S,Xpr>::rootSteps(const IndexId<I>& id) const
     {
-	return mXpr.rootSteps(ptrId);
-    }
-
-    template <SizeT N, SizeT L, SizeT S, class Xpr>
-    template <SizeT L2>
-    constexpr auto SLFor<N,L,S,Xpr>::staticRootSteps(PtrId ptrId) const
-    {
-	return mXpr.template staticRootSteps<L2>(ptrId);
+	return mXpr.rootSteps(id);
     }
 
     template <SizeT N, SizeT L, SizeT S, class Xpr>
@@ -133,18 +121,18 @@ namespace CNORXZ
      *   TFor   *
      ************/
 
-    template <class Xpr>
-    constexpr TFor<Xpr>::TFor(SizeT size, PtrId iptrId, SizeT step, const Xpr& xpr) :
+    template <SizeT L, class Xpr>
+    constexpr TFor<L,Xpr>::TFor(SizeT size, const IndexId<L>& id, SizeT step, const Xpr& xpr) :
 	mSize(size),
-	mIptrId(iptrId),
+	mId(id),
 	mStep(step),
 	mXpr(xpr),
-	mExt(mXpr.rootSteps(mIptrId))
+	mExt(mXpr.rootSteps(mId))
     {}
 
-    template <class Xpr>
+    template <SizeT L, class Xpr>
     template <class PosT1, class PosT2>
-    inline SizeT TFor<Xpr>::operator()(const PosT1& mlast, const PosT2& last) const
+    inline SizeT TFor<L,Xpr>::operator()(const PosT1& mlast, const PosT2& last) const
     {
 	int i = 0;
 #pragma omp parallel shared(mXpr) private(i)
@@ -152,17 +140,16 @@ namespace CNORXZ
 	    auto xpr = mXpr;
 #pragma omp for 
 	    for(i = 0; i < mSize; i++){
-		const UPos I(i);
-		const auto mpos = mlast + mStep * I;
-		const auto pos = last + mExt * I;
+		const auto mpos = mlast + mStep * UPos(i);
+		const auto pos = last + mExt * UPos(i);
 		mXpr(mpos, pos);
 	    }
 	}
 	return 0;
     }
 	
-    template <class Xpr>
-    inline SizeT TFor<Xpr>::operator()() const
+    template <SizeT L, class Xpr>
+    inline SizeT TFor<L,Xpr>::operator()() const
     {
 	int i = 0;
 #pragma omp parallel shared(mXpr) private(i)
@@ -170,26 +157,19 @@ namespace CNORXZ
 	    auto xpr = mXpr;
 #pragma omp for 
 	    for(i = 0; i < static_cast<int>(mSize); i++){
-		const UPos I(i);
-		const auto mpos = mStep * I;
-		const auto pos = mExt * I;
+		const auto mpos = mStep * UPos(i);
+		const auto pos = mExt * UPos(i);
 		mXpr(mpos, pos);
 	    }
 	}
 	return 0;
     }
 
-    template <class Xpr>
-    inline auto TFor<Xpr>::rootSteps(PtrId ptrId) const
+    template <SizeT L, class Xpr>
+    template <SizeT I>
+    inline decltype(auto) TFor<L,Xpr>::rootSteps(const IndexId<I>& id) const
     {
-	return mXpr.rootSteps(ptrId);
-    }
-
-    template <class Xpr>
-    template <SizeT L2>
-    inline auto TFor<Xpr>::staticRootSteps(PtrId ptrId) const
-    {
-	return mXpr.template staticRootSteps<L2>(ptrId);
+	return mXpr.rootSteps(id);
     }
 
 
diff --git a/src/include/ranges/xpr/for.h b/src/include/ranges/xpr/for.h
index 4023108..ec7033a 100644
--- a/src/include/ranges/xpr/for.h
+++ b/src/include/ranges/xpr/for.h
@@ -7,43 +7,29 @@
 
 namespace CNORXZ
 {
-    /*
-    template <class ForT, class Xpr>
-    class ForInterface : public XprInterface<ForInterface<ForT,Xpr>>
-    {
-    public:
-	DEFAULT_MEMBERS(ForInterface);
 
-	inline auto rootSteps(PtrId ptrId) const;
-
-	template <SizeT L>
-	constexpr auto staticRootSteps(PtrId ptrId) const;
-    };
-    */
-    template <class Xpr>
-    class For : public XprInterface<For<Xpr>>
+    template <SizeT L, class Xpr>
+    class For : public XprInterface<For<L,Xpr>>
     {
     public:
 	DEFAULT_MEMBERS(For);
 
-	constexpr For(SizeT size, PtrId iptrId, SizeT step, const Xpr& xpr);
+	constexpr For(SizeT size, const IndexId<L>& id, SizeT step, const Xpr& xpr);
 
 	template <class PosT1, class PosT2>
 	inline SizeT operator()(const PosT1& mlast, const PosT2& last) const;
 	
 	inline SizeT operator()() const;
 
-	inline auto rootSteps(PtrId ptrId) const;
+	template <SizeT I>
+	inline decltype(auto) rootSteps(const IndexId<I>& id) const;
 
-	template <SizeT L>
-	constexpr auto staticRootSteps(PtrId ptrId) const;
-	
     private:
 	SizeT mSize = 0;
-	PtrId mIptrId = 0;
+	IndexId<L> mId;
 	Xpr mXpr;
 	UPos mStep;
-	typedef decltype(mXpr.rootSteps(0)) PosT;
+	typedef decltype(mXpr.rootSteps(mId)) PosT;
 	PosT mExt;
 	
     };
@@ -55,17 +41,15 @@ namespace CNORXZ
     public:
 	DEFAULT_MEMBERS(SLFor);
 
-	constexpr SLFor(PtrId iptrId, const Xpr& xpr);
+	constexpr SLFor(const IndexId<L>& id, const Xpr& xpr);
 
 	template <class PosT1, class PosT2>
 	constexpr SizeT operator()(const PosT1& mlast, const PosT2& last) const;
 	
 	constexpr SizeT operator()() const;
 
-	constexpr auto rootSteps(PtrId ptrId) const;
-
-	template <SizeT L2>
-	constexpr auto staticRootSteps(PtrId ptrId) const;
+	template <SizeT I>
+	constexpr decltype(auto) rootSteps(const IndexId<I>& id) const;
 	
     private:
 
@@ -74,40 +58,38 @@ namespace CNORXZ
 
 	template <SizeT I>
 	constexpr SizeT exec() const;
-	
-	PtrId mIptrId = 0;
+
+	IndexId<L> mId;
 	Xpr mXpr;
-	typedef decltype(mXpr.template staticRootSteps<L>(0)) PosT;
+	typedef decltype(mXpr.RootSteps(mId)) PosT;
 	PosT mExt;
 	
     };
 
 
     // multi-threading
-    template <class Xpr>
-    class TFor : public XprInterface<TFor<Xpr>>
+    template <SizeT L, class Xpr>
+    class TFor : public XprInterface<TFor<L,Xpr>>
     {
     public:
 	DEFAULT_MEMBERS(TFor);
 
-	constexpr TFor(SizeT size, PtrId iptrId, SizeT step, const Xpr& xpr);
+	constexpr TFor(SizeT size, const IndexId<L>& id, SizeT step, const Xpr& xpr);
 
 	template <class PosT1, class PosT2>
 	inline SizeT operator()(const PosT1& mlast, const PosT2& last) const;
 	
 	inline SizeT operator()() const;
 
-	inline auto rootSteps(PtrId ptrId) const;
-
-	template <SizeT L2>
-	inline auto staticRootSteps(PtrId ptrId) const;
+	template <SizeT I>
+	inline decltype(auto) rootSteps(const IndexId<I>& id) const;
 	
     private:
 	SizeT mSize = 0;
-	PtrId mIptrId = 0;
+	IndexId<L> mId = 0;
 	Xpr mXpr;
 	UPos mStep;
-	typedef decltype(mXpr.rootSteps(0)) PosT;
+	typedef decltype(mXpr.rootSteps(mId)) PosT;
 	PosT mExt;
 	
     };
diff --git a/src/include/ranges/xpr/index_id.cc.h b/src/include/ranges/xpr/index_id.cc.h
new file mode 100644
index 0000000..7f9a205
--- /dev/null
+++ b/src/include/ranges/xpr/index_id.cc.h
@@ -0,0 +1,65 @@
+
+#ifndef __cxz_index_id_cc_h__
+#define __cxz_index_id_cc_h__
+
+#include "index_id.h"
+
+namespace CNORXZ
+{
+
+    template <SizeT I>
+    explicit constexpr IndexId<I>::IndexId(PtrId id) :
+	mId(id)
+    {}
+
+    template <SizeT I>
+    constexpr const PtrId& IndexId<I>::id() const
+    {
+	return mId;
+    }
+
+    template <SizeT I>
+    template <SizeT J>
+    constexpr decltype(auto) IndexId<I>::operator==(const IndexId<J>& a) const
+    {
+	if constexpr(I != 0 and J != 0){
+	    if constexpr(I == J){
+		CXZ_ASSERT(mId == a.id(),
+			   "got different index ids for equal static index label");
+		return true;
+	    }
+	    else {
+		CXZ_ASSERT(mId == a.id(),
+			   "got equal index ids for different static index label");
+		return false;
+	    }
+	}
+	else {
+	    return mId == a.id();
+	}
+    }
+
+    template <SizeT I>
+    template <SizeT J>
+    constexpr decltype(auto) IndexId<I>::operator!=(const IndexId<J>& a) const
+    {
+	if constexpr(I != 0 and J != 0){
+	    if constexpr(I == J){
+		CXZ_ASSERT(mId == a.id(),
+			   "got different index ids for equal static index label");
+		return false;
+	    }
+	    else {
+		CXZ_ASSERT(mId == a.id(),
+			   "got equal index ids for different static index label");
+		return true;
+	    }
+	}
+	else {
+	    return mId != a.id();
+	}
+    }
+    
+}
+
+#endif
diff --git a/src/include/ranges/xpr/index_id.h b/src/include/ranges/xpr/index_id.h
new file mode 100644
index 0000000..ed61d68
--- /dev/null
+++ b/src/include/ranges/xpr/index_id.h
@@ -0,0 +1,32 @@
+
+#ifndef __cxz_index_id_h__
+#define __cxz_index_id_h__
+
+#include "base/base.h"
+
+namespace CNORXZ
+{
+
+    // if I == 0: no static comparison; fall back to usual runtime comparison of mId
+    template <SizeT I = 0>
+    class IndexId
+    {
+    private:
+	PtrId mId = 0;
+
+    public:
+	DEFAULT_MEMBERS(IndexId);
+
+	explicit constexpr IndexId(PtrId id);
+
+	constexpr const PtrId& id() const;
+	
+	template <SizeT J>
+	constexpr decltype(auto) operator==(const IndexId<J>& a) const;
+
+	template <SizeT J>
+	constexpr decltype(auto) operator!=(const IndexId<J>& a) const;
+    };
+}
+
+#endif
diff --git a/src/include/ranges/xpr/xpr_base.cc.h b/src/include/ranges/xpr/xpr_base.cc.h
index ccb6dac..8269e70 100644
--- a/src/include/ranges/xpr/xpr_base.cc.h
+++ b/src/include/ranges/xpr/xpr_base.cc.h
@@ -35,7 +35,7 @@ namespace CNORXZ
     }
     
     template <class Xpr>
-    DPos VXpr<Xpr>::vrootSteps(PtrId ptrId) const
+    DPos VXpr<Xpr>::vrootSteps(const IndexId<0>& id) const
     {
 	return DPos(this->rootSteps(ptrId));
     }
@@ -61,15 +61,10 @@ namespace CNORXZ
 	return mC->vexec();
     }
 
-    inline DPos DXpr::rootSteps(PtrId ptrId) const
+    template <SizeT I>
+    inline DPos DXpr::rootSteps(const IndexId<I>& id) const
     {
-	return mC->rootSteps(ptrId);
-    }
-
-    template <SizeT N>
-    inline DPos DXpr::staticRootSteps(PtrId ptrId) const
-    {
-	return this->rootSteps(ptrId);
+	return mC->rootSteps(IndexId<0>(id.id()));
     }
 }
 
diff --git a/src/include/ranges/xpr/xpr_base.h b/src/include/ranges/xpr/xpr_base.h
index 52d53a9..65be564 100644
--- a/src/include/ranges/xpr/xpr_base.h
+++ b/src/include/ranges/xpr/xpr_base.h
@@ -4,6 +4,7 @@
 
 #include "base/base.h"
 #include "pos_type.h"
+#include "index_id.h"
 
 namespace CNORXZ
 {
@@ -22,10 +23,8 @@ namespace CNORXZ
 
 	inline SizeT operator()() const { return THIS()(); }
 
-	inline auto rootSteps(PtrId ptrId) const { return THIS().rootSteps(ptrId); }
-
-	template <SizeT N>
-	constexpr auto staticRootSteps(PtrId ptrId) const { return THIS().template staticRootSteps<N>(ptrId); }
+	template <SizeT I>
+	inline decltype(auto) rootSteps(const IndexId<I>& id) const { return THIS().rootSteps(id); }
     };
 
     class VXprBase
@@ -38,7 +37,7 @@ namespace CNORXZ
 	virtual SizeT vexec(const UPos& mlast, const DPos& last) const = 0;
 	virtual SizeT vexec() const = 0;
 
-	virtual DPos vrootSteps(PtrId ptrId) const = 0;
+	virtual DPos vrootSteps(const IndexId<0>& id) const = 0;
     };
 
     template <class Xpr>
@@ -53,7 +52,7 @@ namespace CNORXZ
 	virtual SizeT vexec(const UPos& mlast, const DPos& last) const override final;
 	virtual SizeT vexec() const override final;
 
-	virtual DPos vrootSteps(PtrId ptrId) const override final;
+	virtual DPos vrootSteps(const IndexId<0>& id) const override final;
     };
 
     class DXpr : public ObjHandle<VXprBase>,
@@ -69,10 +68,8 @@ namespace CNORXZ
 	inline SizeT operator()(const UPos& mlast, const PosT& last) const;
 	inline SizeT operator()() const;
 
-	inline DPos rootSteps(PtrId ptrId) const;
-
-	template <SizeT N>
-	inline DPos staticRootSteps(PtrId ptrId) const; // fallback to rootSteps
+	template <SizeT I>
+	inline DPos rootSteps(const IndexId<I>& id) const;
     };
 }