Demonstrate update_in to manipulate nested struct values

lib/koans/11_structs.ex

@@ -54,4 +54,9 @@ defmodule Structs do
     airline = %Airline{plane: %Plane{maker: :boeing}}
     assert put_in(airline.plane.maker, :airbus) == ___
   end
+
+  koan "Use the update_in macro to modify a nested value" do
+    airline = %Airline{plane: %Plane{maker: :boeing, passengers: 200}}
+    assert update_in(airline.plane.passengers, &(&1 + 2)) == ___
+  end
 end

test/koans/structs_koans_test.exs

@@ -11,6 +11,7 @@ defmodule StructsTests do
       {:multiple, [true, false]},
       {:ok, 22},
       %Structs.Airline{plane: %Structs.Plane{maker: :airbus}, name: "Southwest"},
+      %Structs.Airline{plane: %Structs.Plane{maker: :boeing, passengers: 202}, name: "Southwest"},
     ]
 
     test_all(Structs, answers)