Clean up callback interfaces.

symbols.go

Index: symbols.go
diff --git a/symbols.go b/symbols.go
new file mode 100644
index 0000000000000000000000000000000000000000..813c7afe26a53c67c3d36df491edd35687f9f1e7
--- /dev/null
+++ b/symbols.go
@@ -0,0 +1,13 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package gae
+
+type stopErr struct{}
+
+func (stopErr) Error() string { return "stop iteration" }
+
+// Stop is understood by various services to stop iterative processes. Examples
+// include datastore.Interface.Run's callback.
+var Stop error = stopErr{}