diff --git a/.idea/misc.xml b/.idea/misc.xml
index 76830ba..a6db409 100644
--- a/.idea/misc.xml
+++ b/.idea/misc.xml
@@ -30,6 +30,8 @@
     <cargoProject FILE="$PROJECT_DIR$/reference_cycles/Cargo.toml" />
     <cargoProject FILE="$PROJECT_DIR$/threads/Cargo.toml" />
     <cargoProject FILE="$PROJECT_DIR$/mutex/Cargo.toml" />
+    <cargoProject FILE="$PROJECT_DIR$/oop/Cargo.toml" />
+    <cargoProject FILE="$PROJECT_DIR$/twelve_days_of_christmas/Cargo.toml" />
   </component>
   <component name="ComposerJsonPluginSettings">
     <unboundedVersionInspectionSettings>
diff --git a/.idea/rust.iml b/.idea/rust.iml
index 0650e07..657a62e 100644
--- a/.idea/rust.iml
+++ b/.idea/rust.iml
@@ -122,6 +122,14 @@
       <sourceFolder url="file://$MODULE_DIR$/mutex/examples" isTestSource="false" />
       <sourceFolder url="file://$MODULE_DIR$/mutex/tests" isTestSource="true" />
       <sourceFolder url="file://$MODULE_DIR$/mutex/benches" isTestSource="true" />
+      <sourceFolder url="file://$MODULE_DIR$/oop/src" isTestSource="false" />
+      <sourceFolder url="file://$MODULE_DIR$/oop/examples" isTestSource="false" />
+      <sourceFolder url="file://$MODULE_DIR$/oop/tests" isTestSource="true" />
+      <sourceFolder url="file://$MODULE_DIR$/oop/benches" isTestSource="true" />
+      <sourceFolder url="file://$MODULE_DIR$/twelve_days_of_christmas/src" isTestSource="false" />
+      <sourceFolder url="file://$MODULE_DIR$/twelve_days_of_christmas/examples" isTestSource="false" />
+      <sourceFolder url="file://$MODULE_DIR$/twelve_days_of_christmas/tests" isTestSource="true" />
+      <sourceFolder url="file://$MODULE_DIR$/twelve_days_of_christmas/benches" isTestSource="true" />
       <excludeFolder url="file://$MODULE_DIR$/add/add-one/target" />
       <excludeFolder url="file://$MODULE_DIR$/add/adder/target" />
       <excludeFolder url="file://$MODULE_DIR$/adder/target" />
@@ -141,6 +149,7 @@
       <excludeFolder url="file://$MODULE_DIR$/minigrep_v2/target" />
       <excludeFolder url="file://$MODULE_DIR$/modules/target" />
       <excludeFolder url="file://$MODULE_DIR$/mutex/target" />
+      <excludeFolder url="file://$MODULE_DIR$/oop/target" />
       <excludeFolder url="file://$MODULE_DIR$/rectangles/target" />
       <excludeFolder url="file://$MODULE_DIR$/reference_counting/target" />
       <excludeFolder url="file://$MODULE_DIR$/reference_cycles/target" />
@@ -150,6 +159,7 @@
       <excludeFolder url="file://$MODULE_DIR$/smart_pointers/target" />
       <excludeFolder url="file://$MODULE_DIR$/structs/target" />
       <excludeFolder url="file://$MODULE_DIR$/threads/target" />
+      <excludeFolder url="file://$MODULE_DIR$/twelve_days_of_christmas/target" />
       <excludeFolder url="file://$MODULE_DIR$/variables/target" />
     </content>
     <orderEntry type="inheritedJdk" />
diff --git a/.idea/workspace.xml b/.idea/workspace.xml
index 1826468..5b7d892 100644
--- a/.idea/workspace.xml
+++ b/.idea/workspace.xml
@@ -15,10 +15,14 @@
   <component name="FileEditorManager">
     <leaf>
       <file pinned="false" current-in-tab="true">
-        <entry file="file://$PROJECT_DIR$/mutex/src/main.rs">
+        <entry file="file://$PROJECT_DIR$/oop/src/lib.rs">
           <provider selected="true" editor-type-id="text-editor">
-            <state>
-              <caret column="91" selection-start-column="91" selection-end-column="91" />
+            <state relative-caret-position="784">
+              <caret line="49" column="29" selection-start-line="49" selection-start-column="29" selection-end-line="49" selection-end-column="29" />
+              <folding>
+                <element signature="e#510#511#0" expanded="true" />
+                <element signature="e#537#538#0" expanded="true" />
+              </folding>
             </state>
           </provider>
         </entry>
@@ -36,10 +40,12 @@
     <findStrings>
       <find>ftoc</find>
       <find>expensive_closure(intensity)</find>
+      <find>ordinals</find>
     </findStrings>
     <replaceStrings>
       <replace>f_to_c</replace>
       <replace>expensive_result.value(intensity)</replace>
+      <replace>ORDINALS</replace>
     </replaceStrings>
   </component>
   <component name="Git.Settings">
@@ -48,7 +54,6 @@
   <component name="IdeDocumentHistory">
     <option name="CHANGED_PATHS">
       <list>
-        <option value="$PROJECT_DIR$/variables/src/main.rs" />
         <option value="$PROJECT_DIR$/functions/src/main.rs" />
         <option value="$PROJECT_DIR$/branches/src/main.rs" />
         <option value="$PROJECT_DIR$/ctof/src/main.rs" />
@@ -97,6 +102,9 @@
         <option value="$PROJECT_DIR$/reference_cycles/src/main.rs" />
         <option value="$PROJECT_DIR$/threads/src/main.rs" />
         <option value="$PROJECT_DIR$/mutex/src/main.rs" />
+        <option value="$PROJECT_DIR$/twelve_days_of_christmas/Cargo.toml" />
+        <option value="$PROJECT_DIR$/twelve_days_of_christmas/src/main.rs" />
+        <option value="$PROJECT_DIR$/oop/src/lib.rs" />
       </list>
     </option>
   </component>
@@ -127,17 +135,6 @@
               <item name="rust" type="3d21c010:ScopeViewTreeModel$ProjectNode" />
               <item name="rust" type="2674bda8:ScopeViewTreeModel$GroupNode" />
             </path>
-            <path>
-              <item name="rust" type="3d21c010:ScopeViewTreeModel$ProjectNode" />
-              <item name="rust" type="2674bda8:ScopeViewTreeModel$GroupNode" />
-              <item name="mutex" type="9f88c78c:ScopeViewTreeModel$FileNode" />
-            </path>
-            <path>
-              <item name="rust" type="3d21c010:ScopeViewTreeModel$ProjectNode" />
-              <item name="rust" type="2674bda8:ScopeViewTreeModel$GroupNode" />
-              <item name="mutex" type="9f88c78c:ScopeViewTreeModel$FileNode" />
-              <item name="src" type="9f88c78c:ScopeViewTreeModel$FileNode" />
-            </path>
           </expand>
           <select />
         </subPane>
@@ -169,7 +166,7 @@
   </component>
   <component name="PropertiesComponent">
     <property name="JavaScriptWeakerCompletionTypeGuess" value="true" />
-    <property name="com.android.tools.idea.instantapp.provision.ProvisionBeforeRunTaskProvider.myTimeStamp" value="1549394084677" />
+    <property name="com.android.tools.idea.instantapp.provision.ProvisionBeforeRunTaskProvider.myTimeStamp" value="1549478828057" />
     <property name="javascript.nodejs.core.library.configured.version" value="7.1.0" />
     <property name="js.eslint.eslintPackage" value="$USER_HOME$/.yarn-config/global/node_modules/.bin/eslint" />
     <property name="js.eslint.nodeInterpreter" value="project" />
@@ -197,7 +194,7 @@
       </list>
     </option>
   </component>
-  <component name="RunManager" selected="Cargo Command.Run mutex">
+  <component name="RunManager" selected="Cargo Command.Run twelve_days_of_christmas">
     <configuration name="&lt;template&gt;" type="TestNG" default="true" selected="false">
       <option name="MAIN_CLASS_NAME" />
       <option name="VM_PARAMETERS" value="-ea" />
@@ -256,13 +253,13 @@
       <envs />
       <method v="2" />
     </configuration>
-    <configuration name="Test lib::tests" type="CargoCommandRunConfiguration" factoryName="Cargo Command" temporary="true">
+    <configuration name="Run twelve_days_of_christmas" type="CargoCommandRunConfiguration" factoryName="Cargo Command" temporary="true">
       <option name="channel" value="DEFAULT" />
-      <option name="command" value="test --package interior_mutability --lib tests" />
+      <option name="command" value="run --package twelve_days_of_christmas --bin twelve_days_of_christmas" />
       <option name="allFeatures" value="false" />
       <option name="nocapture" value="false" />
       <option name="backtrace" value="SHORT" />
-      <option name="workingDirectory" value="file://$PROJECT_DIR$/interior_mutability" />
+      <option name="workingDirectory" value="file://$PROJECT_DIR$/twelve_days_of_christmas" />
       <envs />
       <method v="2" />
     </configuration>
@@ -275,11 +272,11 @@
     </configuration>
     <recent_temporary>
       <list>
+        <item itemvalue="Cargo Command.Run twelve_days_of_christmas" />
         <item itemvalue="Cargo Command.Run mutex" />
         <item itemvalue="Cargo Command.Run threads" />
         <item itemvalue="Cargo Command.Run reference_cycles" />
         <item itemvalue="Cargo Command.Run reference_counting" />
-        <item itemvalue="Cargo Command.Test lib::tests" />
       </list>
     </recent_temporary>
   </component>
@@ -319,8 +316,8 @@
     <frame x="1680" y="-410" width="1050" height="1657" extended-state="6" />
     <editor active="true" />
     <layout>
-      <window_info content_ui="combo" id="Project" order="0" sideWeight="0.49586776" visible="true" weight="0.26141885" />
-      <window_info id="Structure" order="1" sideWeight="0.5041322" side_tool="true" visible="true" weight="0.26141885" />
+      <window_info active="true" content_ui="combo" id="Project" order="0" sideWeight="0.49644932" visible="true" weight="0.26141885" />
+      <window_info id="Structure" order="1" sideWeight="0.50355065" side_tool="true" visible="true" weight="0.26141885" />
       <window_info id="Image Layers" order="2" />
       <window_info id="Designer" order="3" />
       <window_info id="Capture Tool" order="4" />
@@ -328,7 +325,7 @@
       <window_info id="Cargo" order="6" sideWeight="0.49791494" weight="0.3286119" />
       <window_info anchor="bottom" id="Message" order="0" />
       <window_info anchor="bottom" id="Find" order="1" weight="0.32979318" />
-      <window_info active="true" anchor="bottom" id="Run" order="2" visible="true" weight="0.29696578" />
+      <window_info anchor="bottom" id="Run" order="2" weight="0.29696578" />
       <window_info anchor="bottom" id="Debug" order="3" weight="0.4" />
       <window_info anchor="bottom" id="Cvs" order="4" weight="0.25" />
       <window_info anchor="bottom" id="Inspection" order="5" weight="0.4" />
@@ -350,20 +347,6 @@
     <option name="version" value="1" />
   </component>
   <component name="editorHistoryManager">
-    <entry file="file://$PROJECT_DIR$/variables/src/main.rs">
-      <provider selected="true" editor-type-id="text-editor">
-        <state relative-caret-position="144">
-          <caret line="9" lean-forward="true" selection-start-line="9" selection-end-line="9" />
-        </state>
-      </provider>
-    </entry>
-    <entry file="file://$PROJECT_DIR$/functions/src/main.rs">
-      <provider selected="true" editor-type-id="text-editor">
-        <state relative-caret-position="128">
-          <caret line="8" column="41" selection-start-line="8" selection-start-column="41" selection-end-line="8" selection-end-column="41" />
-        </state>
-      </provider>
-    </entry>
     <entry file="file://$PROJECT_DIR$/branches/src/main.rs">
       <provider selected="true" editor-type-id="text-editor">
         <state relative-caret-position="160">
@@ -772,8 +755,33 @@
     </entry>
     <entry file="file://$PROJECT_DIR$/mutex/src/main.rs">
       <provider selected="true" editor-type-id="text-editor">
-        <state>
-          <caret column="91" selection-start-column="91" selection-end-column="91" />
+        <state relative-caret-position="640">
+          <caret line="40" selection-start-line="40" selection-end-line="40" />
+        </state>
+      </provider>
+    </entry>
+    <entry file="file://$PROJECT_DIR$/twelve_days_of_christmas/src/main.rs">
+      <provider selected="true" editor-type-id="text-editor">
+        <state relative-caret-position="112">
+          <caret line="7" column="27" lean-forward="true" selection-start-line="7" selection-start-column="27" selection-end-line="7" selection-end-column="27" />
+        </state>
+      </provider>
+    </entry>
+    <entry file="file://$PROJECT_DIR$/twelve_days_of_christmas/Cargo.toml">
+      <provider selected="true" editor-type-id="text-editor">
+        <state relative-caret-position="32">
+          <caret line="2" column="13" selection-start-line="2" selection-start-column="13" selection-end-line="2" selection-end-column="13" />
+        </state>
+      </provider>
+    </entry>
+    <entry file="file://$PROJECT_DIR$/oop/src/lib.rs">
+      <provider selected="true" editor-type-id="text-editor">
+        <state relative-caret-position="784">
+          <caret line="49" column="29" selection-start-line="49" selection-start-column="29" selection-end-line="49" selection-end-column="29" />
+          <folding>
+            <element signature="e#510#511#0" expanded="true" />
+            <element signature="e#537#538#0" expanded="true" />
+          </folding>
         </state>
       </provider>
     </entry>
diff --git a/oop/Cargo.toml b/oop/Cargo.toml
new file mode 100644
index 0000000..cb76d42
--- /dev/null
+++ b/oop/Cargo.toml
@@ -0,0 +1,7 @@
+[package]
+name = "oop"
+version = "0.1.0"
+authors = ["Timothy Warren <twarren@nabancard.com>"]
+edition = "2018"
+
+[dependencies]
diff --git a/oop/src/lib.rs b/oop/src/lib.rs
new file mode 100644
index 0000000..c6d8f42
--- /dev/null
+++ b/oop/src/lib.rs
@@ -0,0 +1,45 @@
+/// Example of encapsulation, as a
+/// requirement of Object Oriented Programming.
+///
+/// The list property is private, eg, encapsulated,
+/// while the object itself handles manipulation of
+/// the list property.
+pub struct AveragedCollection {
+    list: Vec<i32>,
+    average: f64,
+}
+
+impl AveragedCollection {
+    pub fn add(&mut self, value: i32) {
+        self.list.push(value);
+        self.update_average();
+    }
+
+    pub fn remove(&mut self) -> Option<i32> {
+        let result = self.list.pop();
+        match result {
+            Some(value) => {
+                self.update_average();
+                Some(value)
+            },
+            None => None,
+        }
+    }
+
+    pub fn average(&self) -> f64 {
+        self.average
+    }
+
+    fn update_average(&mut self) {
+        let total: i32 = self.list.iter().sum();
+        self.average = total as f64 / self.list.len() as f64;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn it_works() {
+        assert_eq!(2 + 2, 4);
+    }
+}