add a comment about execution of 'run'

src/Swarm/Game.hs

@@ -201,7 +201,23 @@ execConst Run [VString fileName] k r = do
   f <- liftIO $ T.readFile (into fileName)  -- XXX handle file not existing
   case processCmd f of
     Left  err -> error (into err)  -- XXX
-    Right t   -> mkStep r $ In t M.empty (FExec : k)   -- XXX write note
+    Right t   -> mkStep r $ In t M.empty (FExec : k)
+    -- Note, adding FExec to the stack above is correct.  run has the
+    --   type run : String -> Cmd (), i.e. executing (run s) for some
+    --   string s causes it to load *and immediately execute* the
+    --   program in the file.
+    --
+    -- If we instead had
+    --
+    --   load : String -> Cmd (Cmd ())
+    --
+    -- then the code would be the same as for run, EXCEPT that we
+    -- would NOT add the FExec to the stack above.  The fact that
+    -- there are two FExec frames involved in executing 'run' (one to
+    -- execute the run command itself, and one to execute the thing it
+    -- loads) corresponds to the fact that it is equivalent to (in
+    -- pseudo-Haskell syntax) 'join . load'.
+
 execConst Run args k _ = badConst Run args k
 
 badConst :: Const -> [Value] -> Cont -> a