Update, expand, and document plugin tutorial 2

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+(*
+ * This file defines our persistent counter, which we use in the
+ * CountPersistent command.
+ *)
+
+(*
+ * At its core, our persistent counter looks exactly the same as
+ * our non-persistent counter (with a different name to prevent collisions):
+ *)
+let counter = Summary.ref ~name:"persistent_counter" 0
+
+(*
+ * The difference is that we need to declare it as a persistent object
+ * using Libobject.declare_object. To do that, we define a function that
+ * saves the value that is passed to it into the reference we have just defined:
+ *)
+let cache_count (_, v) =
+  counter := v
+
+(*
+ * We then use declare_object to create a function that takes an integer value
+ * (the type our counter refers to) and creates a persistent object from that
+ * value:
+ *)
+let declare_counter : int -> Libobject.obj =
+  let open Libobject in
+  declare_object
+    {
+      (default_object "COUNTER") with
+      cache_function = cache_count;
+      load_function = (fun _ -> cache_count);
+    }
+(*
+ * See Libobject for more information on what other information you
+ * can pass here, and what all of these functions mean.
+ *
+ * For example, if we passed the same thing that we pass to load_function
+ * to open_function, then our last call to Count Persistent in Count.v
+ * would return 4 and not 6.
+ *)
+
+(*
+ * Incrementing our counter looks almost identical:
+ *)
+let increment () =
+  Lib.add_anonymous_leaf (declare_counter (succ !counter))
+(*
+ * except that we must call our declare_counter function to get a persistent
+ * object. We then pass this object to Lib.add_anonymous_leaf.
+ *)
+
+(*
+ * Reading a value does not change at all:
+ *)
+let value () =
+  !counter