diff --git a/gtsam_unstable/nonlinear/Expression-inl.h b/gtsam_unstable/nonlinear/Expression-inl.h
index 47d90a72a..85f89a889 100644
--- a/gtsam_unstable/nonlinear/Expression-inl.h
+++ b/gtsam_unstable/nonlinear/Expression-inl.h
@@ -32,7 +32,15 @@ class Expression;
 typedef std::map<Key, Matrix> JacobianMap;
 
 //-----------------------------------------------------------------------------
+/// The JacobinaTrace class records a tree-structured expression's execution
 struct JacobianTrace {
+  /**
+   *  The Pointer class is a tagged union that obviates the need to create
+   *  a JacobianTrace subclass for Constants and Leaf Expressions. Instead
+   *  the key for the leaf is stored in the space normally used to store a
+   *  JacobianTrace*. Nothing is stored for a Constant.
+   */
+  ///
   class Pointer {
     enum {
       Constant, Leaf, Function
@@ -46,41 +54,43 @@ struct JacobianTrace {
     Pointer() :
         type(Constant) {
     }
+    /// Destructor cleans up pointer if Function
     ~Pointer() {
       if (type == Function)
         delete content.ptr;
     }
+    /// Change pointer to a Leaf Trace
     void setLeaf(Key key) {
       type = Leaf;
       content.key = key;
     }
+    /// Take ownership of pointer to a Function Trace
     void setFunction(JacobianTrace* trace) {
       type = Function;
       content.ptr = trace;
     }
-    // Either add to Jacobians (Leaf) or propagate (Function)
+    // Either insert identity into Jacobians (Leaf) or propagate (Function)
     template<class T>
     void reverseAD(JacobianMap& jacobians) const {
-      if (type == Function)
-        content.ptr->reverseAD(jacobians);
-      else if (type == Leaf) {
+      if (type == Leaf) {
         size_t n = T::Dim();
         jacobians[content.key] = Eigen::MatrixXd::Identity(n, n);
-      }
+      } else if (type == Function)
+        content.ptr->reverseAD(jacobians);
     }
     // Either add to Jacobians (Leaf) or propagate (Function)
     void reverseAD(const Matrix& dTdA, JacobianMap& jacobians) const {
-      if (type == Function)
-        content.ptr->reverseAD(dTdA, jacobians);
-      else if (type == Leaf) {
+      if (type == Leaf) {
         JacobianMap::iterator it = jacobians.find(content.key);
         if (it != jacobians.end())
           it->second += dTdA;
         else
           jacobians[content.key] = dTdA;
-      }
+      } else if (type == Function)
+        content.ptr->reverseAD(dTdA, jacobians);
     }
   };
+  /// Make sure destructor is virtual
   virtual ~JacobianTrace() {
   }
   virtual void reverseAD(JacobianMap& jacobians) const = 0;