singleton: add "maybe" pointer accessor

singleton.hpp

@@ -71,15 +71,25 @@ namespace cruft {
         }
 
 
-        /// returns a reference to sole instantiated value
+        /// Returns a reference to the instantiated value.
         ///
-        /// `instantiate` must have already been called before `get` is called.
+        /// `instantiate` must have already been called before `get` is called
+        /// otherwise the results are undefined.
         static SelfT&
         get (void)
         {
             CHECK (instance);
             return *instance;
         }
+
+
+        /// returns a pointer to the instantiated value if it exists, else we
+        /// return nullptr.
+        static SelfT*
+        maybe (void)
+        {
+            return instance;
+        }
     };
 };