+ * M:N binding of plans to pending requests.
+ * Each pending request can be in a number of plans,
+ * and each plan can have a number of pending requests.
+ * Objects of this type indicate a mapping of a plan to
+ * a particular pending request.
+ *
+ * The corresponding head and tail of the "PE" MDLL
+ * are stored in a `struct GSF_RequestPlan`. (We need
+ * to be able to lookup all pending requests corresponding
+ * to a given plan entry.)
+ *
+ * Similarly head and tail of the "PR" MDLL are stored
+ * with the 'struct GSF_PendingRequest'. (We need
+ * to be able to lookup all plan entries corresponding
+ * to a given pending request.)