Remove arithmetic koans

lib/koans/12_agents.ex

@@ -0,0 +1,36 @@
+defmodule Agents do
+  use Koans
+
+  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)) == :__
+  end
+
+  koan "Update to update the state" do
+    Agent.start_link(fn() -> "Hi there" end, name: __MODULE__)
+
+    Agent.update(__MODULE__, fn(old) ->
+      String.upcase(old)
+    end)
+    assert Agent.get(__MODULE__, &(&1)) == :__
+  end
+
+  koan "Use get_and_update when you need read and change a value in one go" do
+    Agent.start_link(fn() -> ["Milk"] end, name: __MODULE__)
+
+    old_list = Agent.get_and_update(__MODULE__, fn(old) ->
+      {old, ["Bread" | old]}
+    end)
+
+    assert old_list == :__
+    assert Agent.get(__MODULE__, &(&1)) == :__
+  end
+
+  koan "Somebody has to switch off the light at the end of the day" do
+    Agent.start_link(fn() -> ["Milk"] end, name: __MODULE__)
+
+    result = Agent.stop(__MODULE__)
+
+    assert result == :__
+  end
+end