Use bare double underscores instead of atoms

lib/koans/12_agents.ex

@@ -3,7 +3,7 @@ defmodule Agents do
 
   koan "Agents maintain state, so you can ask them about it" do
     Agent.start_link(fn() -> "Hi there" end, name: __MODULE__)
-    assert Agent.get(__MODULE__, &(&1)) == :__
+    assert Agent.get(__MODULE__, &(&1)) == __
   end
 
   koan "Update to update the state" do
@@ -12,7 +12,7 @@ defmodule Agents do
     Agent.update(__MODULE__, fn(old) ->
       String.upcase(old)
     end)
-    assert Agent.get(__MODULE__, &(&1)) == :__
+    assert Agent.get(__MODULE__, &(&1)) == __
   end
 
   koan "Use get_and_update when you need read and change a value in one go" do
@@ -22,8 +22,8 @@ defmodule Agents do
       {old, ["Bread" | old]}
     end)
 
-    assert old_list == :__
-    assert Agent.get(__MODULE__, &(&1)) == :__
+    assert old_list == __
+    assert Agent.get(__MODULE__, &(&1)) == __
   end
 
   koan "Somebody has to switch off the light at the end of the day" do
@@ -31,6 +31,6 @@ defmodule Agents do
 
     result = Agent.stop(__MODULE__)
 
-    assert result == :__
+    assert result == __
   end
 end