diff --git a/tensor.cc b/tensor.cc
index 0328ac6..f86f5b7 100644
--- a/tensor.cc
+++ b/tensor.cc
@@ -2485,6 +2485,13 @@ for(int i=0; i<I.size(); ++i) if(zero_in_index(I[i])) return true;
 return false;
 }
 
+bool  shares_index(const FLATINDEX &I, const FLATINDEX &J)
+{
+for(int i=0; i<I.size(); ++i) for(int j=0; j<J.size(); ++j)
+	if(I[i]==J[j]) return true;
+return false;
+}
+
 
 template class Tensor<double>;
 template class Tensor<std::complex<double> >;
diff --git a/tensor.h b/tensor.h
index 0a1f13e..4680e5b 100644
--- a/tensor.h
+++ b/tensor.h
@@ -231,11 +231,14 @@ 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
 
+
 //useful for negative offsets and 0 index excluded
 bool zero_in_index(const FLATINDEX &);
 bool zero_in_index(const SUPERINDEX &);
 bool zero_in_index(const INDEXMATRIX &, const LA_largeindex row);
 
+bool shares_index(const FLATINDEX &I, const FLATINDEX &J);
+
 FLATINDEX superindex2flat(const SUPERINDEX &I);
 
 template<typename T>