diff --git a/t.cc b/t.cc
index 54110cb..2408964 100644
--- a/t.cc
+++ b/t.cc
@@ -3974,7 +3974,7 @@ cout <<t.dot(u)<<endl;
 }
 
 
-if(1)
+if(0)
 {
 //check full constractions
 int r,n;
@@ -4001,4 +4001,66 @@ cout <<x.dot(y) <<" "<< xf.dot(yf)<< " "<<z<<" "<<zf<<endl;
 }
 
 
+if(0)
+{
+//check symmetrizer/antisymmetrizer with no remaining indices
+int r,n,sym;
+cin>>r>>n>>sym;
+INDEXGROUP shape;
+        {
+        shape.number=r;
+        shape.symmetry= sym;
+        shape.range=n;
+        shape.offset=0;
+        }
+Tensor<double> x(shape); x.randomize(1.);
+//cout <<x;
+Tensor<double> xf=x.flatten();
+
+INDEXLIST il(r);
+for(int i=0; i<r; ++i) il[i]= {i,0};
+Tensor<double> xx = xf.merge_indices(il,sym);
+//cout <<xx;
+cout <<"Error = "<<(xx-x).norm()<<endl;
+
+}
+
+if(1)
+{
+//check symmetrizer/antisymmetrizer in general case
+int r,n,sym;
+cin>>r>>n>>sym;
+NRVec<INDEXGROUP> shape(3);
+	shape[0].number=2; 
+        shape[0].symmetry=0;
+        shape[0].range=n+1;
+        shape[0].offset=0;
+
+        shape[1].number=r;
+        shape[1].symmetry= sym;
+        shape[1].range=n;
+        shape[1].offset=0;
+
+        shape[2].number=2;
+        shape[2].symmetry=0;
+        shape[2].range=n+2;
+        shape[2].offset=0;
+
+
+Tensor<double> x(shape); x.randomize(1.);
+cout <<"x= "<<x.shape;
+Tensor<double> xf=x.flatten(1);
+cout <<"xf= "<<xf.shape;
+Tensor<double> xxx=x.unwind_index_group(1);
+cout <<"xxx= "<<xxx.shape<<endl;
+
+INDEXLIST il(r);
+for(int i=0; i<r; ++i) il[i]= {1+i,0};
+Tensor<double> xx = xf.merge_indices(il,sym);
+cout <<"xx = "<<xx.shape;
+cout <<"Error = "<<(xx-xxx).norm()<<endl;
+
+}
+
+
 }//main
diff --git a/tensor.cc b/tensor.cc
index 8542189..806c9d5 100644
--- a/tensor.cc
+++ b/tensor.cc
@@ -964,6 +964,7 @@ for(int i=0; i<il.size(); ++i)
 	{
 	if(il[i].group<0||il[i].group>=shape.size()) laerror("wrong group number in unwind_indices");
 	if(il[i].index<0||il[i].index>=shape[il[i].group].number)  laerror("wrong index number in unwind_indices");
+	for(int j=0; j<i; ++j) if(il[i]==il[j]) laerror("repeated index in the list");
 	}
 
 //all indices are solo in their groups - permute groups
@@ -1242,6 +1243,7 @@ for(int i=0; i<il1.size(); ++i)
 #ifdef LA_TENSOR_INDEXPOSITION
 	if(rhs1.shape[il1[i].group].upperindex ^ rhs2.shape[il2[i].group].upperindex == false) laerror("can contact only upper with lower index");
 #endif
+	for(int j=0; j<i; ++j) if(il1[i]==il1[j]||il2[i]==il2[j]) laerror("repeated index in the list");
 	}
 
 const Tensor<T> u = conjugate1? (rhs1.unwind_indices(il1)).conjugateme() : rhs1.unwind_indices(il1);
@@ -1650,6 +1652,130 @@ return ind;
 }
 
 
+template<typename T>
+Tensor<T> Tensor<T>::merge_indices(const INDEXLIST &il, int sym) const
+{
+if(il.size()==0) laerror("empty index list for merge_indices");
+if(il.size()==1) return unwind_index(il[0]); //result should be index group of size 1
+
+bool samegroup=true;
+bool isordered=true;
+for(int i=0; i<il.size(); ++i)
+	{
+	if(il[i].group<0||il[i].group>=shape.size()) laerror("wrong group number in merge_indices");
+	if(il[i].index<0||il[i].index>=shape[il[i].group].number)  laerror("wrong index number in merge_indices");
+	for(int j=0; j<i; ++j) if(il[i]==il[j]) laerror("repeated index in the list");
+#ifdef LA_TENSOR_INDEXPOSITION
+	if(shape[il[0].group].upperindex != shape[il[i].group].upperindex == false) laerror("can merge only within lower or upper separately");
+#endif
+	if(shape[il[0].group].range != shape[il[i].group].range) 
+		{
+		std::cout << "indices "<<il[0]<<" and "<<il[i]<< " have ranges "<<shape[il[0].group].range<< " and "<< shape[il[i].group].range <<" respectively\n";
+		laerror("incompatible range in merge_indices");
+		}
+	if(shape[il[0].group].offset != shape[il[i].group].offset) laerror("incompatible offset in merge_indices");
+	if(il[0].group != il[i].group) samegroup=false;
+	if(il[i].index!=i) isordered=false;
+	}
+
+if(samegroup && isordered && il.size()==shape[il[0].group].number) return unwind_index_group(il[0].group);
+
+
+//calculate new shape and flat index permutation
+NRVec<indexgroup> workshape(shape); 
+workshape.copyonwrite();
+NRPerm<int> basicperm(rank());
+
+bitvector was_in_list(rank());
+was_in_list.clear();
+for(int i=0; i<il.size(); ++i)
+	{
+	int fp=flatposition(il[i],shape);
+	was_in_list.set(fp);
+	basicperm[i+1] = 1+fp;
+	if( --workshape[il[i].group].number <0) laerror("inconsistent index list with index group size");
+	}
+int newshapesize=1; //newly created group
+for(int i=0; i<workshape.size(); ++i) if(workshape[i].number>0) ++newshapesize; //this group survived index removal
+
+NRVec<indexgroup> newshape(newshapesize);
+newshape[0].number=il.size();
+newshape[0].symmetry=sym;
+newshape[0].offset=shape[il[0].group].offset;
+newshape[0].range=shape[il[0].group].range;
+#ifdef LA_TENSOR_INDEXPOSITION
+newshape[0].upperindex=shape[il[0].group].upperindex;
+#endif 
+int ii=1;
+for(int i=0; i<workshape.size(); ++i) 
+	if(workshape[i].number>0) 
+		newshape[ii++] = workshape[i];
+int jj=1+il.size();
+for(int i=0; i<rank(); ++i) 
+	if(!was_in_list[i])
+		basicperm[jj++] = 1+i;
+if(!basicperm.is_valid()) laerror("internal error in merge_indices");
+
+//std::cout <<"newshape = "<<newshape<<std::endl;
+//std::cout <<"basicperm = "<<basicperm<<std::endl;
+
+
+//prepare permutation algebra
+PermutationAlgebra<int,T> pa;
+if(sym==0)
+	{
+	pa.resize(1);
+	pa[0].weight=1;
+	pa[0].perm=basicperm;
+	}
+else
+	{
+	PermutationAlgebra<int,int> sa = sym>0 ? symmetrizer<int>(il.size()) : antisymmetrizer<int>(il.size());
+	//std::cout <<"SA = "<<sa<<std::endl;
+	pa.resize(sa.size());
+	for(int i=0; i<sa.size(); ++i)
+		{
+		pa[i].weight = (T) sa[i].weight;
+		pa[i].perm.resize(rank());
+		for(int j=1; j<=il.size(); ++j) pa[i].perm[j] = basicperm[sa[i].perm[j]];
+		for(int j=il.size()+1; j<=rank(); ++j)   pa[i].perm[j] = basicperm[j];
+		}
+	}
+
+//std::cout <<"Use PA = "<<pa<<std::endl;
+
+Tensor<T> r(newshape);
+r.apply_permutation_algebra(*this,pa,false,(T)1/(T)pa.size(),0);
+return r;
+}
+
+template<typename T>
+void Tensor<T>::canonicalize_shape()
+{
+const indexgroup *sh = &(* const_cast<const NRVec<indexgroup> *>(&shape))[0];
+for(int i=0; i<shape.size(); ++i)
+	{
+	if(sh[i].number==1 && sh[i].symmetry!=0) {shape.copyonwrite(); shape[i].symmetry=0;}
+	if(sh[i].symmetry>1 ) {shape.copyonwrite(); shape[i].symmetry=1;}
+	if(sh[i].symmetry<-1) {shape.copyonwrite(); shape[i].symmetry= -1;}
+	}
+}
+
+
+std::ostream & operator<<(std::ostream &s, const INDEX &x)
+{
+s<<x.group<<" "<<x.index;
+return s;
+}
+
+std::istream & operator>>(std::istream &s, INDEX &x)
+{ 
+s>>x.group>>x.index;
+return s;
+}
+        
+
+
 
 template class Tensor<double>;
 template class Tensor<std::complex<double> >;
diff --git a/tensor.h b/tensor.h
index 53cda74..a23c498 100644
--- a/tensor.h
+++ b/tensor.h
@@ -41,15 +41,11 @@
 
 //TODO:
 //@@@contraction inside one tensor - compute resulting shape, loopover the shape, create index into the original tensor + loop over the contr. index, do the summation, store result
-//@@@ will need to store vector of INDEX to the original tensor for the result's flatindex
-//@@@ will not be particularly efficient
-//
-//maybe optional negative range for beta spin handling in some cases of fourindex-tensor conversions
-//
+//@@@ will need to store vector of INDEX to the original tensor for the result's flatindex, will not be particularly efficient
+//@@@?maybe optional negative range for beta spin handling in some cases of fourindex-tensor conversions
 //@@@?general permutation of individual indices - check the indices in sym groups remain adjacent, calculate result's shape, loopover the result and permute using unwind_callback
-//@@@? apply_permutation_algebra if result should be symmetric/antisymmetric in such a way to compute only the nonredundant part
-//@@@symetrizace a antisymetrizace skupiny indexu - jak efektivneji nez pres permutationalgebra?
-//
+//@@@ is that needed? we can flatten the relevant groups and permute index groups alternatively - maybe implement on high level this way for convenience
+
 
 
 //do not distinguish covariant/contravariant indices
@@ -160,9 +156,14 @@ struct  INDEX
 {
 int group;
 int index;
+bool operator==(const INDEX &rhs) const {return group==rhs.group && index==rhs.index;};
 };
 typedef NRVec<INDEX> INDEXLIST; //collection of several indices
 
+std::ostream & operator<<(std::ostream &s, const INDEX &x);
+std::istream & operator>>(std::istream &s, INDEX &x);
+
+
 int flatposition(int group, int index, const NRVec<indexgroup> &shape);
 int flatposition(const INDEX &i, const NRVec<indexgroup> &shape); //position of that index in FLATINDEX
 INDEX indexposition(int flatindex, const NRVec<indexgroup> &shape); //inverse to flatposition
@@ -188,12 +189,12 @@ public:
 	//constructors
 	Tensor() : myrank(-1) {};
 	explicit Tensor(const T &x) : myrank(0), data(1) {data[0]=x;}; //scalar
-	Tensor(const NRVec<indexgroup> &s) : shape(s) { data.resize(calcsize()); calcrank();}; //general tensor
-	Tensor(const NRVec<indexgroup> &s, const NRVec<INDEXNAME> &newnames) : shape(s), names(newnames) { data.resize(calcsize());  calcrank(); if(names.size()!=myrank && names.size()!=0) laerror("bad number of index names");}; //general tensor
-	Tensor(const NRVec<indexgroup> &s, const NRVec<T> &mydata) : shape(s) { LA_largeindex dsize=calcsize(); calcrank(); if(mydata.size()!=dsize) laerror("inconsistent data size with shape"); data=mydata;} 
-	Tensor(const NRVec<indexgroup> &s, const NRVec<T> &mydata, const NRVec<INDEXNAME> &newnames) : shape(s), names(newnames) { LA_largeindex dsize=calcsize(); calcrank(); if(mydata.size()!=dsize) laerror("inconsistent data size with shape"); data=mydata;  if(names.size()!=myrank && names.size()!=0) laerror("bad number of index names");} 
-	Tensor(const indexgroup &g) {shape.resize(1); shape[0]=g; data.resize(calcsize()); calcrank();}; //tensor with a single index group
-	Tensor(const indexgroup &g, const NRVec<INDEXNAME> &newnames) : names(newnames) {shape.resize(1); shape[0]=g; data.resize(calcsize()); calcrank(); if(names.size()!=myrank && names.size()!=0) laerror("bad number of index names");}; //tensor with a single index group
+	Tensor(const NRVec<indexgroup> &s) : shape(s) { data.resize(calcsize()); calcrank(); canonicalize_shape();}; //general tensor
+	Tensor(const NRVec<indexgroup> &s, const NRVec<INDEXNAME> &newnames) : shape(s), names(newnames) { data.resize(calcsize());  calcrank(); canonicalize_shape(); if(names.size()!=myrank && names.size()!=0) laerror("bad number of index names");}; //general tensor
+	Tensor(const NRVec<indexgroup> &s, const NRVec<T> &mydata) : shape(s) { LA_largeindex dsize=calcsize(); calcrank(); canonicalize_shape(); if(mydata.size()!=dsize) laerror("inconsistent data size with shape"); data=mydata;} 
+	Tensor(const NRVec<indexgroup> &s, const NRVec<T> &mydata, const NRVec<INDEXNAME> &newnames) : shape(s), names(newnames) { LA_largeindex dsize=calcsize(); calcrank(); canonicalize_shape(); if(mydata.size()!=dsize) laerror("inconsistent data size with shape"); data=mydata;  if(names.size()!=myrank && names.size()!=0) laerror("bad number of index names");} 
+	Tensor(const indexgroup &g) {shape.resize(1); shape[0]=g; data.resize(calcsize()); calcrank(); canonicalize_shape();}; //tensor with a single index group
+	Tensor(const indexgroup &g, const NRVec<INDEXNAME> &newnames) : names(newnames) {shape.resize(1); shape[0]=g; data.resize(calcsize()); calcrank(); canonicalize_shape(); if(names.size()!=myrank && names.size()!=0) laerror("bad number of index names");}; //tensor with a single index group
 	Tensor(const Tensor &rhs): myrank(rhs.myrank), shape(rhs.shape), groupsizes(rhs.groupsizes), cumsizes(rhs.cumsizes), data(rhs.data), names(rhs.names) {};
 	Tensor(int xrank, const NRVec<indexgroup> &xshape, const NRVec<LA_largeindex> &xgroupsizes,  const  NRVec<LA_largeindex> xcumsizes, const  NRVec<T> &xdata) :  myrank(xrank), shape(xshape), groupsizes(xgroupsizes), cumsizes(xcumsizes), data(xdata) {};
 	Tensor(int xrank, const NRVec<indexgroup> &xshape, const NRVec<LA_largeindex> &xgroupsizes,  const  NRVec<LA_largeindex> xcumsizes, const  NRVec<T> &xdata, const NRVec<INDEXNAME>  &xnames) :  myrank(xrank), shape(xshape), groupsizes(xgroupsizes), cumsizes(xcumsizes), data(xdata), names(xnames) {};
@@ -212,6 +213,7 @@ public:
 	void defaultnames() {names.resize(rank()); for(int i=0; i<rank(); ++i) sprintf(names[i].name,"i%03d",i);}
 	int rank() const {return myrank;};
 	int calcrank(); //is computed from shape
+	void canonicalize_shape(); 
 	LA_largeindex calcsize(); //set redundant data and return total size
 	LA_largeindex size() const {return data.size();};
 	void copyonwrite() {shape.copyonwrite(); groupsizes.copyonwrite(); cumsizes.copyonwrite(); data.copyonwrite(); names.copyonwrite();};
@@ -316,6 +318,7 @@ public:
 
 	Tensor merge_index_groups(const NRVec<int> &groups) const; 
 	Tensor flatten(int group= -1) const; //split and uncompress a given group or all of them, leaving flat index order the same
+	Tensor merge_indices(const INDEXLIST &il, int symmetry=0) const;	//opposite to flatten (merging with optional symmetrization/antisymmetrization and compression)
 
 	NRVec<NRMat<T> > Tucker(typename LA_traits<T>::normtype thr=1e-12, bool inverseorder=true); //HOSVD-Tucker decomposition, return core tensor in *this, flattened 
 	Tensor inverseTucker(const NRVec<NRMat<T> > &x, bool inverseorder=true) const; //rebuild the original tensor from Tucker