diff --git a/cpp/NonlinearFactor.h b/cpp/NonlinearFactor.h
index 4dde65720..6c2771177 100644
--- a/cpp/NonlinearFactor.h
+++ b/cpp/NonlinearFactor.h
@@ -139,6 +139,10 @@ namespace gtsam {
 		NonlinearFactor1() {
 		}
 
+		inline const Key& key() const {
+			return key_;
+		}
+
 		/**
 		 *  Constructor
 		 *  @param z measurement
diff --git a/cpp/NonlinearOptimizer-inl.h b/cpp/NonlinearOptimizer-inl.h
index 2bd01a8d5..c9bde7155 100644
--- a/cpp/NonlinearOptimizer-inl.h
+++ b/cpp/NonlinearOptimizer-inl.h
@@ -46,8 +46,8 @@ namespace gtsam {
 	/* ************************************************************************* */
 	// Constructors without the solver
 	/* ************************************************************************* */
-	template<class G, class C, class L, class S>
-	NonlinearOptimizer<G, C, L, S>::NonlinearOptimizer(shared_graph graph,
+	template<class G, class C, class L, class S, class W>
+	NonlinearOptimizer<G, C, L, S, W>::NonlinearOptimizer(shared_graph graph,
 			shared_config config, shared_solver solver, double lambda) :
 		graph_(graph), config_(config), error_(graph->error(
 				*config)), lambda_(lambda), solver_(solver) {
@@ -56,8 +56,8 @@ namespace gtsam {
 	/* ************************************************************************* */
 	// linearize and optimize
 	/* ************************************************************************* */
-	template<class G, class C, class L, class S>
-	VectorConfig NonlinearOptimizer<G, C, L, S>::linearizeAndOptimizeForDelta() const {
+	template<class G, class C, class L, class S, class W>
+	VectorConfig NonlinearOptimizer<G, C, L, S, W>::linearizeAndOptimizeForDelta() const {
 		L linearized = solver_->linearize(*graph_, *config_);
 		return solver_->optimize(linearized);
 	}
@@ -65,8 +65,8 @@ namespace gtsam {
 	/* ************************************************************************* */
 	// One iteration of Gauss Newton
 	/* ************************************************************************* */
-	template<class G, class C, class L, class S>
-	NonlinearOptimizer<G, C, L, S> NonlinearOptimizer<G, C, L, S>::iterate(
+	template<class G, class C, class L, class S, class W>
+	NonlinearOptimizer<G, C, L, S, W> NonlinearOptimizer<G, C, L, S, W>::iterate(
 			verbosityLevel verbosity) const {
 		// linearize and optimize
 		VectorConfig delta = linearizeAndOptimizeForDelta();
@@ -82,7 +82,7 @@ namespace gtsam {
 		if (verbosity >= CONFIG)
 			newConfig->print("newConfig");
 
-		NonlinearOptimizer newOptimizer = NonlinearOptimizer(graph_, newConfig, solver_);
+		NonlinearOptimizer newOptimizer = NonlinearOptimizer(graph_, newConfig, solver_, lambda_);
 
 		if (verbosity >= ERROR)
 			cout << "error: " << newOptimizer.error_ << endl;
@@ -91,18 +91,22 @@ namespace gtsam {
 	}
 
 	/* ************************************************************************* */
-	template<class G, class C, class L, class S>
-	NonlinearOptimizer<G, C, L, S> NonlinearOptimizer<G, C, L, S>::gaussNewton(
+	template<class G, class C, class L, class S, class W>
+	NonlinearOptimizer<G, C, L, S, W> NonlinearOptimizer<G, C, L, S, W>::gaussNewton(
 			double relativeThreshold, double absoluteThreshold,
 			verbosityLevel verbosity, int maxIterations) const {
+		static W writer(error_);
+
 		// linearize, solve, update
 		NonlinearOptimizer next = iterate(verbosity);
 
+		writer.write(next.error_);
+
 		// check convergence
 		bool converged = gtsam::check_convergence(relativeThreshold,
 				absoluteThreshold, error_, next.error_, verbosity);
 
-		// return converged state or iterate
+				// return converged state or iterate
 		if (converged)
 			return next;
 		else
@@ -115,8 +119,8 @@ namespace gtsam {
 	// optimizer if error decreased or recurse with a larger lambda if not.
 	// TODO: in theory we can't infinitely recurse, but maybe we should put a max.
 	/* ************************************************************************* */
-	template<class G, class C, class L, class S>
-	NonlinearOptimizer<G, C, L, S> NonlinearOptimizer<G, C, L, S>::try_lambda(
+	template<class G, class C, class L, class S, class W>
+	NonlinearOptimizer<G, C, L, S, W> NonlinearOptimizer<G, C, L, S, W>::try_lambda(
 			const L& linear, verbosityLevel verbosity, double factor) const {
 
 		if (verbosity >= TRYLAMBDA)
@@ -153,8 +157,8 @@ namespace gtsam {
 	/* ************************************************************************* */
 	// One iteration of Levenberg Marquardt
 	/* ************************************************************************* */
-	template<class G, class C, class L, class S>
-	NonlinearOptimizer<G, C, L, S> NonlinearOptimizer<G, C, L, S>::iterateLM(
+	template<class G, class C, class L, class S, class W>
+	NonlinearOptimizer<G, C, L, S, W> NonlinearOptimizer<G, C, L, S, W>::iterateLM(
 			verbosityLevel verbosity, double lambdaFactor) const {
 
 		// maybe show output
@@ -175,8 +179,8 @@ namespace gtsam {
 	}
 
 	/* ************************************************************************* */
-	template<class G, class C, class L, class S>
-	NonlinearOptimizer<G, C, L, S> NonlinearOptimizer<G, C, L, S>::levenbergMarquardt(
+	template<class G, class C, class L, class S, class W>
+	NonlinearOptimizer<G, C, L, S, W> NonlinearOptimizer<G, C, L, S, W>::levenbergMarquardt(
 			double relativeThreshold, double absoluteThreshold,
 			verbosityLevel verbosity, int maxIterations, double lambdaFactor) const {
 
diff --git a/cpp/NonlinearOptimizer.h b/cpp/NonlinearOptimizer.h
index c9ea6c407..92796b00b 100644
--- a/cpp/NonlinearOptimizer.h
+++ b/cpp/NonlinearOptimizer.h
@@ -14,6 +14,12 @@
 
 namespace gtsam {
 
+	class NullOptimizerWriter {
+	public:
+		NullOptimizerWriter(double error) {}
+		virtual void write(double error) {}
+	};
+
 	/**
 	 * The class NonlinearOptimizer encapsulates an optimization state.
 	 * Typically it is instantiated with a NonlinearFactorGraph and an initial config
@@ -32,7 +38,7 @@ namespace gtsam {
 	 * nonlinear cost function around the current estimate, and the second one optimize the
 	 * linearized system using various methods.
 	 */
-	template<class G, class T, class L = GaussianFactorGraph, class S = Factorization<G, T> >
+	template<class G, class T, class L = GaussianFactorGraph, class S = Factorization<G, T>, class Writer = NullOptimizerWriter>
 	class NonlinearOptimizer {
 	public: