class Explain {
 public:
-    static void explainStages(PlanExecutor* exec,
+    static void explainStages(const PlanExecutor* exec,
                               ExplainCommon::Verbosity verbosity,
                               BSONObjBuilder* out);

     static BSONObj statsToBSON(const PlanStageStats& stats,
                                ExplainCommon::Verbosity verbosity = ExplainCommon::EXEC_STATS);

     static void statsToBSON(const PlanStageStats& stats,
                             BSONObjBuilder* bob,
                             ExplainCommon::Verbosity verbosity = ExplainCommon::EXEC_STATS);

-    static std::string getPlanSummary(PlanExecutor* exec);
+    static std::string getPlanSummary(const PlanExecutor* exec);

-    static std::string getPlanSummary(PlanStage* root);
+    static std::string getPlanSummary(const PlanStage* root);

-    static void getSummaryStats(PlanExecutor* exec, PlanSummaryStats* statsOut);
+    static void getSummaryStats(const PlanExecutor* exec, PlanSummaryStats* statsOut);
 };