Use triple underscore instead of double

test/support/sample_koan.ex

@@ -2,7 +2,7 @@ defmodule SampleKoan do
   use Koans
 
   koan "Thinking more than once" do
-    assert 3 == :__
-    assert 4 == :__
+    assert 3 == ___
+    assert 4 == ___
   end
 end