diff --git a/gtsam/hybrid/HybridGaussianFactorGraph.cpp b/gtsam/hybrid/HybridGaussianFactorGraph.cpp
index e03e58f7a..b5344a988 100644
--- a/gtsam/hybrid/HybridGaussianFactorGraph.cpp
+++ b/gtsam/hybrid/HybridGaussianFactorGraph.cpp
@@ -65,7 +65,7 @@ static GaussianMixtureFactor::Sum &addGaussian(
   if (sum.empty()) {
     GaussianFactorGraph result;
     result.push_back(factor);
-    sum = GaussianMixtureFactor::Sum(
+    return GaussianMixtureFactor::Sum(
         GaussianMixtureFactor::GraphAndConstant(result, 0.0));
 
   } else {
@@ -75,12 +75,19 @@ static GaussianMixtureFactor::Sum &addGaussian(
       result.push_back(factor);
       return GaussianMixtureFactor::GraphAndConstant(result, graph_z.constant);
     };
-    sum = sum.apply(add);
+    return sum.apply(add);
   }
-  return sum;
 }
 
 /* ************************************************************************ */
+// TODO(dellaert): At the time I though "Sum" was a good name, but coming back
+// to it after a while I think "SumFrontals" is better.it's a terrible name.
+// Also, the implementation is inconsistent. I think we should just have a
+// virtual method in HybridFactor that adds to the "Sum" object, like
+// addGaussian. Finally, we need to document why deferredFactors need to be
+// added last, which I would undo if possible.
+// Implementation-wise, it's probably more efficient to first collect the
+// discrete keys, and then loop over all assignments to populate a vector.
 GaussianMixtureFactor::Sum HybridGaussianFactorGraph::SumFrontals() const {
   // sum out frontals, this is the factor on the separator
   gttic(sum);
@@ -89,8 +96,8 @@ GaussianMixtureFactor::Sum HybridGaussianFactorGraph::SumFrontals() const {
   std::vector<GaussianFactor::shared_ptr> deferredFactors;
 
   for (auto &f : factors_) {
+    // TODO(dellaert): just use a virtual method defined in HybridFactor.
     if (f->isHybrid()) {
-      // TODO(dellaert): just use a virtual method defined in HybridFactor.
       if (auto gm = boost::dynamic_pointer_cast<GaussianMixtureFactor>(f)) {
         sum = gm->add(sum);
       }
diff --git a/gtsam/hybrid/HybridGaussianFactorGraph.h b/gtsam/hybrid/HybridGaussianFactorGraph.h
index db0687642..ed3ada2ff 100644
--- a/gtsam/hybrid/HybridGaussianFactorGraph.h
+++ b/gtsam/hybrid/HybridGaussianFactorGraph.h
@@ -18,10 +18,10 @@
 
 #pragma once
 
+#include <gtsam/hybrid/GaussianMixtureFactor.h>
 #include <gtsam/hybrid/HybridFactor.h>
 #include <gtsam/hybrid/HybridFactorGraph.h>
 #include <gtsam/hybrid/HybridGaussianFactor.h>
-#include <gtsam/hybrid/GaussianMixtureFactor.h>
 #include <gtsam/inference/EliminateableFactorGraph.h>
 #include <gtsam/inference/FactorGraph.h>
 #include <gtsam/inference/Ordering.h>
@@ -122,9 +122,9 @@ class GTSAM_EXPORT HybridGaussianFactorGraph
   /// @name Adding factors.
   /// @{
 
-  using Base::reserve;
   using Base::add;
   using Base::push_back;
+  using Base::reserve;
 
   /// Add a Jacobian factor to the factor graph.
   void add(JacobianFactor&& factor);
@@ -236,8 +236,15 @@ class GTSAM_EXPORT HybridGaussianFactorGraph
    */
   const Ordering getHybridOrdering() const;
 
-  /// Compute a DecisionTree<Key, GaussianFactorGraph> with the marginal for
-  /// each discrete assignment.
+  /**
+   * @brief Create a decision tree of factor graphs out of this hybrid factor
+   * graph.
+   *
+   * For example, if there are two mixture factors, one with a discrete key A
+   * and one with a discrete key B, then the decision tree will have two levels,
+   * one for A and one for B. The leaves of the tree will be the Gaussian
+   * factors that have only continuous keys.
+   */
   GaussianMixtureFactor::Sum SumFrontals() const;
 
   /// @}