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diff --git a/Firestore/Source/Model/FSTDocumentDictionary.mm b/Firestore/Source/Model/FSTDocumentDictionary.mm
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+++ b/Firestore/Source/Model/FSTDocumentDictionary.mm
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+/*
+ * Copyright 2017 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#import "Firestore/Source/Model/FSTDocumentDictionary.h"
+
+#import "Firestore/Source/Model/FSTDocumentKey.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@implementation FSTImmutableSortedDictionary (FSTMaybeDocumentDictionary)
+
++ (instancetype)maybeDocumentDictionary {
+  // Immutable dictionaries are contravariant in their value type, so just return a
+  // FSTDocumentDictionary here.
+  return [FSTDocumentDictionary documentDictionary];
+}
+
++ (instancetype)documentDictionary {
+  static FSTDocumentDictionary *singleton;
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
+    singleton = [FSTDocumentDictionary dictionaryWithComparator:FSTDocumentKeyComparator];
+  });
+  return singleton;
+}
+
+@end
+
+NS_ASSUME_NONNULL_END