diff --git a/js/src/route.js b/js/src/route.js
index c5ba6ad..cdbb76e 100644
--- a/js/src/route.js
+++ b/js/src/route.js
@@ -415,9 +415,19 @@ function subtract(o1, o2, subtractSuccessesOpt) {
 	  if (!k2) {
 	    return;
 	  }
-	  updatedK = walkWild(key, k2, w1, walkFlipped);
+	  // There is an entry in r2 but not r1 for our key.
+	  updatedK =
+	    is_emptyTrie(w1) ? emptyTrie :
+	    is_keyOpen(key) ? walk(rwildseq(w1), k2) :
+	    is_keyClose(key) ? ((w1 instanceof $WildcardSequence) ? walk(w1.trie, k2) : emptyTrie) :
+	    walk(w1, k2);
 	} else if (!k2) {
-	  updatedK = walkWild(key, k1, w2, walk);
+	  // There is an entry in r1 but not r2 for our key.
+	  updatedK =
+	    is_emptyTrie(w2) ? k1 :
+	    is_keyOpen(key) ? walk(k1, rwildseq(w2)) :
+	    is_keyClose(key) ? ((w2 instanceof $WildcardSequence) ? walk(k1, w2.trie) : k1) :
+	    walk(k1, w2);
 	} else {
 	  updatedK = walk(k1, k2);
 	}
@@ -476,16 +486,6 @@ function subtract(o1, o2, subtractSuccessesOpt) {
 
     return target;
   }
-
-  function walkWild(key, k, w, walker) {
-    if (is_emptyTrie(w)) return k;
-    if (is_keyOpen(key)) return walker(k, rwildseq(w));
-    if (is_keyClose(key)) {
-      if (w instanceof $WildcardSequence) return walker(k, w.trie);
-      return k;
-    }
-    return walker(k, w);
-  }
 }
 
 // Returns null on failed match, otherwise the appropriate success
diff --git a/js/test/test-patch.js b/js/test/test-patch.js
index d6c5f07..d028658 100644
--- a/js/test/test-patch.js
+++ b/js/test/test-patch.js
@@ -92,6 +92,22 @@ describe('patch sequencing', function () {
 		      '                2::: nothing'],
     		     [' < $Observe < 1 2 > > >{true}']);
   });
+
+  it('works for longer chains of asserts and retracts', function () {
+    var rawPatch =
+	Patch.assert(1)
+	.andThen(Patch.retract(2))
+	.andThen(Patch.retract(3))
+	.andThen(Patch.assert(4))
+	.andThen(Patch.retract(99));
+      checkPrettyPatch(rawPatch,
+		       [' 1 >{true}',
+		        ' 4 >{true}'],
+		       [' 2 >{true}',
+		        ' 3 >{true}',
+		        ' 99 >{true}']);
+
+  });
 });
 
 describe('patch lifting', function () {
diff --git a/js/test/test-route.js b/js/test/test-route.js
index 5450ffe..14e41af 100644
--- a/js/test/test-route.js
+++ b/js/test/test-route.js
@@ -180,6 +180,18 @@ describe("subtraction", function () {
 		     "   3 ★...> >{true}",
 		     "     >::: nothing"]);
   });
+
+  it("should be idempotent if the subtrahend doesn't overlap the minuend", function () {
+    checkPrettyTrie(r.compilePattern(true, 1),
+		    [' 1 >{true}']);
+    checkPrettyTrie(r.subtract(r.compilePattern(true, 1),
+			       r.compilePattern(true, 2)),
+		    [' 1 >{true}']);
+    checkPrettyTrie(r.subtract(r.compilePattern(true, 1),
+			       r.compilePattern(true, 2),
+			       function (v1, v2) { return null; }),
+		    [' 1 >{true}']);
+  });
 });
 
 describe("subtract after union", function () {