diff --git a/src/SIL.Machine.Morphology.HermitCrab/PhonologicalRules/NarrowAnalysisRewriteRuleSpec.cs b/src/SIL.Machine.Morphology.HermitCrab/PhonologicalRules/NarrowAnalysisRewriteRuleSpec.cs
index 9d913b06c..6380eaab1 100644
--- a/src/SIL.Machine.Morphology.HermitCrab/PhonologicalRules/NarrowAnalysisRewriteRuleSpec.cs
+++ b/src/SIL.Machine.Morphology.HermitCrab/PhonologicalRules/NarrowAnalysisRewriteRuleSpec.cs
@@ -40,6 +40,9 @@ RewriteSubrule subrule
         private void Unapply(Match<Word, ShapeNode> targetMatch, Range<ShapeNode> range, VariableBindings varBindings)
         {
             ShapeNode curNode = IsTargetEmpty ? range.Start : range.End;
+            ShapeNode targetNode = range.Start;
+            int targetPos = 0;
+            bool zipper = true;
             foreach (
                 Constraint<Word, ShapeNode> constraint in _analysisRhs.Children.CastToConstraints(
                     "The target (left-hand side) of a rewrite rule"
@@ -49,14 +52,33 @@ Constraint<Word, ShapeNode> constraint in _analysisRhs.Children.CastToConstraint
                 FeatureStruct fs = constraint.FeatureStruct.Clone();
                 if (varBindings != null)
                     fs.ReplaceVariables(varBindings);
-                curNode = targetMatch.Input.Shape.AddAfter(curNode, fs, true);
+                if (
+                    zipper
+                    && targetPos < _targetCount
+                    && constraint.Type() == HCFeatureSystem.Segment
+                    && targetNode.Type() == HCFeatureSystem.Segment
+                    && !fs.HasVariables // This can be removed when Union is fixed.
+                    && !targetNode.Annotation.FeatureStruct.HasVariables
+                )
+                {
+                    // Union fs into the existing node's feature structure.
+                    targetNode.Annotation.FeatureStruct.Union(fs);
+                    targetNode = targetNode.Next;
+                    targetPos++;
+                }
+                else
+                {
+                    // Add an optional node for fs.
+                    curNode = targetMatch.Input.Shape.AddAfter(curNode, fs, true);
+                    zipper = false;
+                }
             }
 
-            curNode = range.Start;
-            for (int i = 0; i < _targetCount; i++)
+            // Make the rest of the existing nodes optional.
+            for (int i = targetPos; i < _targetCount; i++)
             {
-                curNode.Annotation.Optional = true;
-                curNode = curNode.Next;
+                targetNode.Annotation.Optional = true;
+                targetNode = targetNode.Next;
             }
         }
     }
diff --git a/tests/SIL.Machine.Morphology.HermitCrab.Tests/PhonologicalRules/RewriteRuleTests.cs b/tests/SIL.Machine.Morphology.HermitCrab.Tests/PhonologicalRules/RewriteRuleTests.cs
index 5e0bf867b..be7135bc3 100644
--- a/tests/SIL.Machine.Morphology.HermitCrab.Tests/PhonologicalRules/RewriteRuleTests.cs
+++ b/tests/SIL.Machine.Morphology.HermitCrab.Tests/PhonologicalRules/RewriteRuleTests.cs
@@ -553,6 +553,63 @@ public void MultipleMergeRules()
         AssertMorphsEqual(morpher.ParseWord("bttbtt"), "27");
     }
 
+    [Test]
+    public void MergeWithBoundary()
+    {
+        var merge = new RewriteRule
+        {
+            Name = "merge",
+            Lhs = Pattern<Word, ShapeNode>
+                .New()
+                .Annotation(Character(Table3, "b"))
+                .Annotation(Character(Table3, "+"))
+                .Annotation(Character(Table3, "b"))
+                .Value,
+        };
+        merge.Subrules.Add(
+            new RewriteSubrule
+            {
+                Rhs = Pattern<Word, ShapeNode>
+                    .New()
+                    .Annotation(Character(Table3, "p"))
+                    .Annotation(Character(Table3, "i"))
+                    .Value,
+            }
+        );
+        Morphophonemic.PhonologicalRules.Add(merge);
+
+        var morpher = new Morpher(TraceManager, Language, 1);
+        AssertMorphsEqual(morpher.ParseWord("apia"), "39");
+    }
+
+    [Test]
+    public void AlphaAgreementMergeFindsAbbaFromApa()
+    {
+        FeatureStruct alphaVoicedConsonant = FeatureStruct
+            .New(Language.PhonologicalFeatureSystem)
+            .Symbol(HCFeatureSystem.Segment)
+            .Symbol("cons+")
+            .Feature("vd")
+            .EqualToVariable("a")
+            .Value;
+        var merge = new RewriteRule
+        {
+            Name = "alphaMerge",
+            Lhs = Pattern<Word, ShapeNode>
+                .New()
+                .Annotation(alphaVoicedConsonant)
+                .Annotation(alphaVoicedConsonant)
+                .Value,
+        };
+        merge.Subrules.Add(
+            new RewriteSubrule { Rhs = Pattern<Word, ShapeNode>.New().Annotation(Character(Table3, "p")).Value }
+        );
+        Morphophonemic.PhonologicalRules.Add(merge);
+
+        var morpher = new Morpher(TraceManager, Language);
+        AssertMorphsEqual(morpher.ParseWord("apa"), "40");
+    }
+
     [Test]
     public void ExpandRules()
     {