Demonstrate put_in to manipulate nested struct values

lib/koans/11_structs.ex

@@ -45,4 +45,13 @@ defmodule Structs do
 
     assert Map.fetch(silvia, :age) == ___
   end
+
+  defmodule Airline do
+    defstruct plane: %Plane{}, name: "Southwest"
+  end
+
+  koan "Use the put_in macro to replace a nested value" do
+    airline = %Airline{plane: %Plane{maker: :boeing}}
+    assert put_in(airline.plane.maker, :airbus) == ___
+  end
 end

test/koans/structs_koans_test.exs

@@ -10,6 +10,7 @@ defmodule StructsTests do
       33,
       {:multiple, [true, false]},
       {:ok, 22},
+      %Structs.Airline{plane: %Structs.Plane{maker: :airbus}, name: "Southwest"},
     ]
 
     test_all(Structs, answers)