diff --git a/src/include/base/to_string.cc.h b/src/include/base/to_string.cc.h
index dac3678..34e2adf 100644
--- a/src/include/base/to_string.cc.h
+++ b/src/include/base/to_string.cc.h
@@ -55,6 +55,12 @@ namespace CNORXZ
 	      } );
     }
 
+    template <typename T, typename S>
+    String ToString<std::pair<T,S>>::func(const std::pair<T,S>& p)
+    {
+	return String("(") + toString(p.first) + "," + toString(p.second) + ")";
+    }
+
     template <typename T>
     String toString(const T& a)
     {
diff --git a/src/include/base/to_string.h b/src/include/base/to_string.h
index 08f055b..f5cd2e5 100644
--- a/src/include/base/to_string.h
+++ b/src/include/base/to_string.h
@@ -35,7 +35,13 @@ namespace CNORXZ
     {
 	static String func(const Tuple<Ts...>& t);
     };
-    
+
+    template <typename T, typename S>
+    struct ToString<std::pair<T,S>>
+    {
+	static String func(const std::pair<T,S>& t);
+    };
+
     template <>
     struct ToString<DType>
     {
diff --git a/src/include/ranges/urange.cc.h b/src/include/ranges/urange.cc.h
index 89a0bfe..4964619 100644
--- a/src/include/ranges/urange.cc.h
+++ b/src/include/ranges/urange.cc.h
@@ -233,7 +233,7 @@ namespace CNORXZ
     {
 	std::sort(mSpace.begin(), mSpace.end(), std::less<MetaType>());
 	auto itdupl = std::adjacent_find(mSpace.begin(), mSpace.end());
-	CXZ_ASSERT(itdupl == mSpace.end(), "found duplicate: " << *itdupl);
+	CXZ_ASSERT(itdupl == mSpace.end(), "found duplicate: " << toString(*itdupl));
     }
 
         
diff --git a/src/opt/hdf5/include/h5_group.cc.h b/src/opt/hdf5/include/h5_group.cc.h
index 554bc44..58d8f11 100644
--- a/src/opt/hdf5/include/h5_group.cc.h
+++ b/src/opt/hdf5/include/h5_group.cc.h
@@ -16,7 +16,7 @@ namespace CNORXZ
 
 	template <typename... Ts>
 	Group& Group::addTable(const String& name, const ArrayBase<Tuple<Ts...>>& data,
-			       const RangePtr& fields)
+			       const Vector<String>& fnames)
 	{
 	    CXZ_ASSERT(this->isOpen(), "tried to extend closed group");
 	    Vector<String> nvec({name});
@@ -25,7 +25,7 @@ namespace CNORXZ
 	    mCont.extend(extr);
 	    auto ii = mCont.begin();
 	    ii.at(dvec); // 'at' returns YIndex&, so cannot use it inline...
-	    auto tab = std::make_shared<STable<Ts...>>(name, this, fields);
+	    auto tab = std::make_shared<STable<Ts...>>(name, this, fnames);
 	    for(auto& d: data){
 		tab->appendRecord(d);
 	    }
diff --git a/src/opt/hdf5/include/h5_group.h b/src/opt/hdf5/include/h5_group.h
index a13cd02..ce487c3 100644
--- a/src/opt/hdf5/include/h5_group.h
+++ b/src/opt/hdf5/include/h5_group.h
@@ -34,7 +34,7 @@ namespace CNORXZ
 
 	    template <typename... Ts>
 	    Group& addTable(const String& name, const ArrayBase<Tuple<Ts...>>& data,
-			    const RangePtr& fields);
+			    const Vector<String>& fnames);
 	    
 	protected:
 	    MArray<ContentPtr> mCont;
diff --git a/src/opt/hdf5/include/h5_table.cc.h b/src/opt/hdf5/include/h5_table.cc.h
index cc42228..0ec028d 100644
--- a/src/opt/hdf5/include/h5_table.cc.h
+++ b/src/opt/hdf5/include/h5_table.cc.h
@@ -18,13 +18,19 @@ namespace CNORXZ
 	}
 
 	template <typename... Ts>
-	STable<Ts...>::STable(const String& name, const ContentBase* _parent, const RangePtr& fields) :
+	STable<Ts...>::STable(const String& name, const ContentBase* _parent,
+			      const Vector<String>& fnames) :
 	    Table(name, _parent)
 	{
 	    constexpr SizeT N = sizeof...(Ts);
 	    if(mFields == nullptr){
-		CXZ_ASSERT(fields != nullptr, "field names have to be initialized");
-		mFields = fields;
+		CXZ_ASSERT(fnames.size() != 0, "field names have to be initialized");
+		Vector<FieldID> fields(fnames.size());
+		for(SizeT i = 0; i != fields.size(); ++i){
+		    fields[i].first = i;
+		    fields[i].second = fnames[i];
+		}
+		mFields = URangeFactory<FieldID>(fields).create();
 	    }
 	    CXZ_ASSERT(mFields->size() == sizeof...(Ts), "expected tuple of size = " << mFields->size()
 		       << ", got: " << sizeof...(Ts));
@@ -66,9 +72,9 @@ namespace CNORXZ
 	    if(mRecords == nullptr){
 		mRecords = appr;
 		Vector<const char*> fields(mFields->size());
-		auto fr = std::dynamic_pointer_cast<URange<String>>(mFields);
+		auto fr = std::dynamic_pointer_cast<URange<FieldID>>(mFields);
 		for(auto fi = fr->begin(); fi != fr->end(); ++fi){
-		    fields[fi.lex()] = (*fi).c_str();
+		    fields[fi.lex()] = (*fi).second.c_str();
 		}
 		H5TBmake_table(mName.c_str(), mParent->id(), mName.c_str(), mFields->size(),
 			       mRecords->size(), sizeof(t), fields.data(), mOffsets.data(),
diff --git a/src/opt/hdf5/include/h5_table.h b/src/opt/hdf5/include/h5_table.h
index 18a6476..5ae5ca4 100644
--- a/src/opt/hdf5/include/h5_table.h
+++ b/src/opt/hdf5/include/h5_table.h
@@ -11,6 +11,8 @@ namespace CNORXZ
 	class Table : public ContentBase
 	{
 	public:
+	    typedef std::pair<SizeT,String> FieldID;
+	    
 	    DEFAULT_MEMBERS(Table);
 	    Table(const String& name, const ContentBase* _parent);
 	    ~Table();
@@ -41,7 +43,7 @@ namespace CNORXZ
 	{
 	public:
 	    DEFAULT_MEMBERS(STable);
-	    STable(const String& name, const ContentBase* _parent, const RangePtr& fields);
+	    STable(const String& name, const ContentBase* _parent, const Vector<String>& fnames);
 
 	    STable& appendRecord(const Tuple<Ts...>& t);
 	    STable& appendRecord(const MArray<Tuple<Ts...>>& t);
diff --git a/src/opt/hdf5/lib/h5_table.cc b/src/opt/hdf5/lib/h5_table.cc
index 8ecd4dd..c68b75f 100644
--- a/src/opt/hdf5/lib/h5_table.cc
+++ b/src/opt/hdf5/lib/h5_table.cc
@@ -20,11 +20,12 @@ namespace CNORXZ
 		SizeT typesize = 0;
 		H5TBget_field_info(mParent->id(), mName.c_str(), fieldsptr.data(), sizes.data(),
 				   offsets.data(), &typesize);
-		Vector<String> fields(nfields);
+		Vector<FieldID> fields(nfields);
 		for(SizeT i = 0; i != nfields; ++i){
-		    fields[i] = fieldsptr[i];
+		    fields[i].first = i;
+		    fields[i].second = fieldsptr[i];
 		}
-		mFields = URangeFactory<String>( std::move(fields) ).create();
+		mFields = URangeFactory<FieldID>( std::move(fields) ).create();
 		mSizes = MArray<SizeT>(mFields, std::move(sizes));
 		mOffsets = MArray<SizeT>(mFields, std::move(offsets));
 		this->open();
@@ -87,7 +88,12 @@ namespace CNORXZ
 	Table& Table::initFieldNames(const Vector<String>& fnames)
 	{
 	    CXZ_ASSERT(mFields == nullptr, "fields already initialized");
-	    mFields = URangeFactory<String>(fnames).create();
+	    Vector<FieldID> fields(fnames.size());
+	    for(SizeT i = 0; i != fields.size(); ++i){
+		fields[i].first = i;
+		fields[i].second = fnames[i];
+	    }
+	    mFields = URangeFactory<FieldID>(fields).create();
 	    return *this;
 	}
     }
diff --git a/src/opt/hdf5/tests/h5_basic_unit_test.cc b/src/opt/hdf5/tests/h5_basic_unit_test.cc
index 1399651..172c707 100644
--- a/src/opt/hdf5/tests/h5_basic_unit_test.cc
+++ b/src/opt/hdf5/tests/h5_basic_unit_test.cc
@@ -40,7 +40,7 @@ namespace
 	{
 	    mFileName = testh5file;
 	    mGrps = { "gr1", "gr2" };
-	    mFs = URangeFactory<String>(Vector<String>({"field1","second","real"})).create();
+	    mFs = {"field1","second","real"};
 	    Vector<Tuple<SizeT,Int,Double>> v
 		( { {0, -6, 3.141},
 		    {3, -8, 0.789},
@@ -55,7 +55,7 @@ namespace
 	String mFileName;
 	Vector<String> mGrps;
 
-	RangePtr mFs;
+	Vector<String> mFs;
 	MArray<Tuple<SizeT,Int,Double>> mTabA; 
     };