diff --git a/lib/desugaring.rb b/lib/desugaring.rb
index 8d52908..c5cd243 100644
--- a/lib/desugaring.rb
+++ b/lib/desugaring.rb
@@ -21,7 +21,10 @@ def all_the_sugar(recipients, event, message)
   # Copy the contents of the previous method here and remove this sugar.
   #
   def desugared_poetry(recipients, event, message)
-    implement_me!
+    mail(message,
+      to: recipients.map(&:email),
+      subject: "You’re invited to #{event.title} on #{event.date}"
+    )
   end
 
   # Ruby allows you to pass arguments identified by name instead of just by position. They are really just
@@ -36,7 +39,10 @@ def desugared_poetry(recipients, event, message)
   # Copy the contents of the previous method here and remove this sugar.
   #
   def desugared_named_args(recipients, event, message)
-    implement_me!
+    mail(message, {
+      to: recipients.map(&:email),
+      subject: "You’re invited to #{event.title} on #{event.date}"
+    })
   end
 
   # Ruby’s general syntax for hashes is `{key => value, key => value, ...}`. Because it is so common to use
@@ -51,7 +57,10 @@ def desugared_named_args(recipients, event, message)
   # Copy the contents of the previous method here and remove this sugar.
   #
   def desugared_symbol_keys(recipients, event, message)
-    implement_me!
+    mail(message, {
+      :to => recipients.map(&:email),
+      :subject => "You’re invited to #{event.title} on #{event.date}"
+    })
   end
 
   # You may be wondering how `map(&:email)` works. When you precede the last argument of a method call with
@@ -70,7 +79,10 @@ def desugared_symbol_keys(recipients, event, message)
   # Copy the contents of the previous method here and remove this sugar.
   #
   def desugared_attr_proc(recipients, event, message)
-    implement_me!
+    mail(message, {
+      :to => recipients.map { |x| x.email },
+      :subject => "You’re invited to #{event.title} on #{event.date}"
+    })
   end
 
   # You may recall from the Ruby koans that when you put `#{something}` in a `"`-delimited string, Ruby will
@@ -88,7 +100,10 @@ def desugared_attr_proc(recipients, event, message)
   # Copy the contents of the previous method here and remove this sugar.
   #
   def desugared_interpolation(recipients, event, message)
-    implement_me!
+    mail(message, {
+      :to => recipients.map { |x| x.email },
+      :subject => "You’re invited to " + event.title.to_s + " on " + event.date.to_s
+    })
   end
 
   # Ruby tracks local variables lexically at compile time. Wherever you say `x = y`, the compiler assumes that
@@ -110,7 +125,10 @@ def desugared_interpolation(recipients, event, message)
   # (Think: which names are local variables, and which are not?)
   #
   def desugared_implicit_self(recipients, event, message)
-    implement_me!
+    self.mail(message, {
+      :to => recipients.map { |x| x.email },
+      :subject => "You’re invited to " + event.title.to_s + " on " + event.date.to_s
+    })
   end
 
   # In Ruby, unlike Python, there are no properties distinct from method calls. When you say `x.y`, you are
@@ -131,7 +149,10 @@ def desugared_implicit_self(recipients, event, message)
   # but structurally quite similar!
   #
   def desugared_implicit_parens(recipients, event, message)
-    implement_me!
+    self.mail(message, {
+      :to => recipients.map { |x| x.email() },
+      :subject => "You’re invited to " + event.title().to_s() + " on " + event.date().to_s()
+    })
   end
 
   # In Ruby, every value is an object and every action is a method call. That includes operators. A binary
@@ -153,7 +174,13 @@ def desugared_implicit_parens(recipients, event, message)
   #   get added before the things on the right. (a + b + c) means ((a + b) + c), NOT (a + (b + c)).
   #
   def desugared_operators(recipients, event, message)
-    implement_me!
+    self.mail(message, {
+      :to => recipients.map { |x| x.email() },
+      :subject => ((("You’re invited to ")
+                                        .+(event.title().to_s()))
+                                          .+(" on "))
+                                            .+(event.date().to_s())
+    })
   end
 
   # Compare that to the version at the top.
diff --git a/lib/door.rb b/lib/door.rb
index 0f99a74..55bbe7b 100644
--- a/lib/door.rb
+++ b/lib/door.rb
@@ -16,29 +16,21 @@ class Door
     end
   end
 
-  aasm :deadbolt_lock, namespace: :deadbolt do
-    state :unlocked, initial: true
-    state :locked
-
-    event :lock_deadbolt do
-      transitions to: :locked
-    end
-
-    event :unlock_deadbolt do
-      transitions to: :unlocked
+  def self.generate_lock(lock_type)
+    aasm "#{lock_type}_lock".to_sym, namespace: lock_type.to_sym do
+      state :unlocked, initial: true
+      state :locked
+
+      event "lock_#{lock_type}".to_sym do
+        transitions to: :locked
+      end
+
+      event "unlock_#{lock_type}".to_sym do
+        transitions to: :unlocked
+      end
     end
   end
 
-  aasm :knob_lock, namespace: :knob do
-    state :unlocked, initial: true
-    state :locked
+["deadbolt", "knob"].each { |lock_type| generate_lock lock_type }
 
-    event :lock_knob do
-      transitions to: :locked
-    end
-
-    event :unlock_knob do
-      transitions to: :unlocked
-    end
-  end
 end
diff --git a/lib/retail_transaction.rb b/lib/retail_transaction.rb
index 1bfa0e3..545e1f0 100644
--- a/lib/retail_transaction.rb
+++ b/lib/retail_transaction.rb
@@ -39,6 +39,7 @@ def paid?
     state :processing_payment
     state :payment_declined
     state :settled
+    state :refunded
 
     event :check_out do
       transitions from: :ringing_up, to: :collecting_payment,
@@ -61,5 +62,9 @@ def paid?
     event :payment_declined do
       transitions from: :processing_payment, to: :payment_declined
     end
+
+    event :refund do
+      transitions from: :settled, to: :refunded
+    end
   end
 end
diff --git a/test/retail_transaction_test.rb b/test/retail_transaction_test.rb
index 823cbfe..2b69fad 100644
--- a/test/retail_transaction_test.rb
+++ b/test/retail_transaction_test.rb
@@ -103,6 +103,10 @@
       assert_equal false, tx.settled?
       assert_equal true,  tx.payment_declined?
     end
+
+    it "cannot be refunded" do
+      assert_equal false, tx.may_refund?
+    end
   end
 
   describe "with declined payment" do
@@ -138,6 +142,10 @@
       assert_equal false, tx.payment_declined?
       assert_equal true,  tx.processing_payment?
     end
+
+    it "cannot be refunded" do
+      assert_equal false, tx.may_refund?
+    end
   end
 
   describe "that is settled" do
@@ -152,5 +160,28 @@
     it "cannot be reopened" do
       assert_invalid_transition { tx.reopen! }
     end
+
+    it "can be refunded" do
+      assert_equal true, tx.may_refund?
+    end
+  end
+
+  describe "that has been refunded" do
+    before(:each) do
+      tx.add_item("bobcat")
+      tx.check_out!
+      tx.payment_info = "15 cents and a nail"
+      tx.process_payment!
+      tx.payment_authorized!
+      tx.refund!
+    end
+
+    it "cannot be refunded again" do
+      assert_equal false, tx.may_refund?
+    end
+
+    it "cannot be reopened" do
+      assert_equal false, tx.may_reopen?
+    end
   end
 end