example lava and wall block in playground

HW11/Minigrid/minigrid/envs/mchw11.py

@@ -8,13 +8,10 @@ from minigrid.core.grid import Grid
 from minigrid.core.mission import MissionSpace
 from minigrid.envs.lavaslippery import LavaSlipperyEnv
 from minigrid.core.world_object import (
-    Ball,
-    Box,
-    Key,
     Slippery,
     Lava,
     Goal,
-    Point
+    Wall
  )
 
 from minigrid.minigrid_env import MiniGridEnv, is_slippery
@@ -40,8 +37,7 @@ class MCHW11Env(LavaSlipperyEnv):
         self.disable_random_start()
 
         self.put_obj(Lava(),4,3)
-        self.put_obj(Lava(),7,3)
-        self.put_obj(Lava(),1,9)
+        self.put_obj(Wall(),9,3)