diff --git a/cpp/Tensor2.h b/cpp/Tensor2.h
index 87e2b3962..36bc5ce75 100644
--- a/cpp/Tensor2.h
+++ b/cpp/Tensor2.h
@@ -10,41 +10,45 @@
 
 namespace tensors {
 
-	/** Rank 2 Tensor */
-	template<int N1, int N2>
-	class Tensor2 {
-	protected:
-		Tensor1<N1> T[N2];
+/** Rank 2 Tensor */
+template<int N1, int N2>
+class Tensor2 {
+protected:
+	Tensor1<N1> T[N2];
 
-	public:
+public:
 
-		/** default constructor */
-		Tensor2() {
-		}
+	/** default constructor */
+	Tensor2() {
+	}
 
-		/* construct from data */
-		Tensor2(const double data[N2][N1]) {
-			for (int j = 0; j < N2; j++)
-				T[j] = Tensor1<N1> (data[j]);
-		}
+	/* construct from data */
+	Tensor2(const double data[N2][N1]) {
+		for (int j = 0; j < N2; j++)
+			T[j] = Tensor1<N1> (data[j]);
+	}
 
-		/** construct from expression */
-		template<class A, char I, char J>
-		Tensor2(const Tensor2Expression<A, Index<N1, I> , Index<N2, J> >& a) {
-			for (int j = 0; j < N2; j++)
-				T[j] = a(j);
-		}
+	/** construct from expression */
+	template<class A, char I, char J>
+	Tensor2(const Tensor2Expression<A, Index<N1, I> , Index<N2, J> >& a) {
+		for (int j = 0; j < N2; j++)
+			T[j] = a(j);
+	}
 
-		double operator()(int i, int j) const {
-			return T[j](i);
-		}
+	const double & operator()(int i, int j) const {
+		return T[j](i);
+	}
 
-		/** convert to expression */
-		template<char I, char J> Tensor2Expression<Tensor2, Index<N1, I> , Index<
-				N2, J> > operator()(Index<N1, I> i, Index<N2, J> j) const {
-			return Tensor2Expression<Tensor2, Index<N1, I> , Index<N2, J> > (*this);
-		}
-	};
+	double & operator()(int i, int j) {
+		return T[j](i);
+	}
+
+	/** convert to expression */
+	template<char I, char J> Tensor2Expression<Tensor2, Index<N1, I> , Index<
+			N2, J> > operator()(Index<N1, I> i, Index<N2, J> j) const {
+		return Tensor2Expression<Tensor2, Index<N1, I> , Index<N2, J> > (*this);
+	}
+};
 
 } // namespace tensors