need helper functionality for tests to scrap existing log contents

...this is necessary whenever the mocked facility covered
by log matching is managed automatically as singleton,
because then other test cases will leave garbage
in the log

src/lib/test/event-log.hpp

@@ -643,6 +643,34 @@ namespace test{
         }
       
       
+      /** */
+      EventLog&
+      clear()
+        {
+          UNIMPLEMENTED ("clear log contents while retaining just the original Header-ID");
+        }
+      
+      /** */
+      EventLog&
+      clear (string alteredLogID)
+        {
+          UNIMPLEMENTED ("clear log contents and reset Header-ID");
+        }
+      
+      EventLog&
+      clear (const char* alteredLogID)
+        {
+          return clear (string{alteredLogID});
+        }
+      
+      template<class X>
+      EventLog&
+      clear (const X *const obj)
+        {
+          UNIMPLEMENTED ("clear log contents and reset Header-ID");
+        }
+      
+      
       
       /* ==== Logging API ==== */
       

tests/library/test/test-event-log-test.cpp

@@ -67,6 +67,7 @@ namespace test{
           verify_eventLogging();
           verify_genericLogging();
           verify_regExpMatch();
+          verify_logPurging();
         }
       
       
@@ -351,6 +352,25 @@ namespace test{
           // argument match must cover all arguments...
           CHECK (log.ensureNot("spam").argMatch("bacon|^spam"));
         }
+      
+      
+      void
+      verify_logPurging ()
+        {
+          EventLog log("obnoxious");
+          log.create("spam").create("spam").create("spam");
+          CHECK (log.verify("spam").after("obnoxious"));
+          
+          log.clear();
+          CHECK (log.ensureNot("spam"));
+          CHECK (log.verify("obnoxious").type("EventLogHeader").on("obnoxious"));
+          
+          log.warn("eggs");
+          log.clear("unbearable");
+          CHECK (log.ensureNot("eggs"));
+          CHECK (log.ensureNot("obnoxious"));
+          CHECK (log.verify("unbearable").type("EventLogHeader").on(&log));
+        }
     };
   
   LAUNCHER (TestEventLog_test, "unit common");