From 8db2cd17fc8d03e43fe943ee2b565dd6e23982ee Mon Sep 17 00:00:00 2001
From: dellaert <dellaert@cc.gatech.edu>
Date: Sun, 5 Oct 2014 13:41:20 +0200
Subject: [PATCH] Finished constant Trace and *everything* just works!!!
 Amazing :-)

---
 gtsam_unstable/nonlinear/Expression-inl.h | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/gtsam_unstable/nonlinear/Expression-inl.h b/gtsam_unstable/nonlinear/Expression-inl.h
index 16489ea80..cfd0bf7f2 100644
--- a/gtsam_unstable/nonlinear/Expression-inl.h
+++ b/gtsam_unstable/nonlinear/Expression-inl.h
@@ -205,10 +205,22 @@ public:
     return Augmented<T>(constant_);
   }
 
+  /// Trace structure for reverse AD
+  typedef typename ExpressionNode<T>::Trace BaseTrace;
+  struct Trace: public BaseTrace {
+    T t;
+    /// Return value and derivatives
+    virtual Augmented<T> augmented(const Matrix& H) const {
+      // Base case: just return value and empty map
+      return Augmented<T>(t);
+    }
+  };
+
   /// Construct an execution trace for reverse AD
-  virtual boost::shared_ptr<typename ExpressionNode<T>::Trace> reverse(
-      const Values& values) const {
-    return boost::shared_ptr<typename ExpressionNode<T>::Trace>();
+  virtual boost::shared_ptr<BaseTrace> reverse(const Values& values) const {
+    boost::shared_ptr<Trace> trace = boost::make_shared<Trace>();
+    trace->t = constant_;
+    return trace;
   }
 };