Use bare double underscores instead of atoms

lib/koans/03_tuples.ex

@@ -2,34 +2,34 @@ defmodule Tuples do
   use Koans
 
   koan "Tuples have a size" do
-    assert tuple_size({:a, :b, :c}) == :__
+    assert tuple_size({:a, :b, :c}) == __
   end
 
   koan "Tuples can contain different things" do
-    assert {:a, 1, "hi"} == :__
+    assert {:a, 1, "hi"} == __
   end
 
   koan "You can pull out individual elements" do
-    assert elem({:a, "hi"}, 1) == :__
+    assert elem({:a, "hi"}, 1) == __
   end
 
   koan "You can change individual elements of a tuple" do
-    assert put_elem({:a, "hi"}, 1, "bye") == :__
+    assert put_elem({:a, "hi"}, 1, "bye") == __
   end
 
   koan "You can also simply extend a tuple with new stuff" do
-    assert Tuple.insert_at({:a, "hi"}, 1, :new_thing) == :__
+    assert Tuple.insert_at({:a, "hi"}, 1, :new_thing) == __
   end
 
   koan "Add things at the end" do
-    assert Tuple.append({"Huey", "Dewey"}, "Louie") == :__
+    assert Tuple.append({"Huey", "Dewey"}, "Louie") == __
   end
 
   koan "Or remove them" do
-    assert Tuple.delete_at({:this, :is, :not, :awesome}, 2) == :__
+    assert Tuple.delete_at({:this, :is, :not, :awesome}, 2) == __
   end
 
   koan "Turn it into a list in case you need it" do
-    assert Tuple.to_list({:this, :can, :be, :a, :list}) == :__
+    assert Tuple.to_list({:this, :can, :be, :a, :list}) == __
   end
 end