diff --git a/gtsam/discrete/DiscreteFactor.h b/gtsam/discrete/DiscreteFactor.h
index 6fa074379..62f1e064b 100644
--- a/gtsam/discrete/DiscreteFactor.h
+++ b/gtsam/discrete/DiscreteFactor.h
@@ -136,9 +136,6 @@ class GTSAM_EXPORT DiscreteFactor : public Factor {
   /**
    * @brief Multiply in a DiscreteFactor and return the result as
    * DiscreteFactor, both via shared pointers.
-   *
-   * @param df DiscreteFactor shared_ptr
-   * @return DiscreteFactor::shared_ptr
    */
   virtual DiscreteFactor::shared_ptr multiply(
       const DiscreteFactor::shared_ptr& df) const = 0;
@@ -170,7 +167,7 @@ class GTSAM_EXPORT DiscreteFactor : public Factor {
    * 
    * @return DiscreteFactor::shared_ptr 
    */
-  DiscreteFactor::shared_ptr scale() const;
+  virtual DiscreteFactor::shared_ptr scale() const;
 
   /**
    * Get the number of non-zero values contained in this factor.
diff --git a/gtsam_unstable/discrete/AllDiff.h b/gtsam_unstable/discrete/AllDiff.h
index 1180abad4..67a6ffd18 100644
--- a/gtsam_unstable/discrete/AllDiff.h
+++ b/gtsam_unstable/discrete/AllDiff.h
@@ -72,6 +72,11 @@ class GTSAM_UNSTABLE_EXPORT AllDiff : public Constraint {
   /// Partially apply known values, domain version
   Constraint::shared_ptr partiallyApply(
       const Domains&) const override;
+
+  // Scale just returns the same constraint.
+  DiscreteFactor::shared_ptr scale() const override {
+    return std::make_shared<AllDiff>(*this);
+  }
 };
 
 }  // namespace gtsam
diff --git a/gtsam_unstable/discrete/BinaryAllDiff.h b/gtsam_unstable/discrete/BinaryAllDiff.h
index e96bfdfde..21d5f1642 100644
--- a/gtsam_unstable/discrete/BinaryAllDiff.h
+++ b/gtsam_unstable/discrete/BinaryAllDiff.h
@@ -96,6 +96,11 @@ class BinaryAllDiff : public Constraint {
   AlgebraicDecisionTree<Key> errorTree() const override {
     throw std::runtime_error("BinaryAllDiff::error not implemented");
   }
+
+  // Scale just returns the same constraint.
+  DiscreteFactor::shared_ptr scale() const override {
+    return std::make_shared<BinaryAllDiff>(*this);
+  }
 };
 
 }  // namespace gtsam
diff --git a/gtsam_unstable/discrete/Constraint.h b/gtsam_unstable/discrete/Constraint.h
index 328fabbea..91bcdbc20 100644
--- a/gtsam_unstable/discrete/Constraint.h
+++ b/gtsam_unstable/discrete/Constraint.h
@@ -29,7 +29,9 @@ class Domain;
 using Domains = std::map<Key, Domain>;
 
 /**
- * Base class for constraint factors
+ * Base class for constraint factors.
+ * This class is used to represent constraints on discrete variables.
+ * The values are always either 0 or 1, with at least one 1.
  * Derived classes include SingleValue, BinaryAllDiff, and AllDiff.
  */
 class GTSAM_UNSTABLE_EXPORT Constraint : public DiscreteFactor {
@@ -80,6 +82,16 @@ class GTSAM_UNSTABLE_EXPORT Constraint : public DiscreteFactor {
   /// Partially apply known values, domain version
   virtual shared_ptr partiallyApply(const Domains&) const = 0;
 
+  /// Multiply in a DecisionTreeFactor.
+  DecisionTreeFactor operator*(const DecisionTreeFactor& df) const override {
+    throw std::logic_error("Constraint::operator* not implemented");
+  }
+
+  /// Multiply with scalar just returns the constraint.
+  DiscreteFactor::shared_ptr operator*(double /* s*/) const override {
+    throw std::logic_error("Constraint::operator* not implemented");
+  }
+
   /// Multiply factors, DiscreteFactor::shared_ptr edition
   DiscreteFactor::shared_ptr multiply(
       const DiscreteFactor::shared_ptr& df) const override {
@@ -104,6 +116,8 @@ class GTSAM_UNSTABLE_EXPORT Constraint : public DiscreteFactor {
     return toDecisionTreeFactor().sum(keys);
   }
 
+  double max() const override { return 1.0; }
+
   DiscreteFactor::shared_ptr max(size_t nrFrontals) const override {
     return toDecisionTreeFactor().max(nrFrontals);
   }
diff --git a/gtsam_unstable/discrete/Domain.h b/gtsam_unstable/discrete/Domain.h
index 6ce846201..b45b1f23e 100644
--- a/gtsam_unstable/discrete/Domain.h
+++ b/gtsam_unstable/discrete/Domain.h
@@ -114,6 +114,11 @@ class GTSAM_UNSTABLE_EXPORT Domain : public Constraint {
 
   /// Partially apply known values, domain version
   Constraint::shared_ptr partiallyApply(const Domains& domains) const override;
+
+  // Scale just returns the same constraint.
+  DiscreteFactor::shared_ptr scale() const override {
+    return std::make_shared<Domain>(*this);
+  }
 };
 
 }  // namespace gtsam
diff --git a/gtsam_unstable/discrete/SingleValue.h b/gtsam_unstable/discrete/SingleValue.h
index 3df1209b8..152a8f51b 100644
--- a/gtsam_unstable/discrete/SingleValue.h
+++ b/gtsam_unstable/discrete/SingleValue.h
@@ -77,6 +77,11 @@ class GTSAM_UNSTABLE_EXPORT SingleValue : public Constraint {
   /// Partially apply known values, domain version
   Constraint::shared_ptr partiallyApply(
       const Domains& domains) const override;
+
+  // Scale just returns the same constraint.
+  DiscreteFactor::shared_ptr scale() const override {
+    return std::make_shared<SingleValue>(*this);
+  }
 };
 
 }  // namespace gtsam