diff --git a/.idea/workspace.xml b/.idea/workspace.xml
index 8270e60..2ce6851 100644
--- a/.idea/workspace.xml
+++ b/.idea/workspace.xml
@@ -2,8 +2,6 @@
 <project version="4">
   <component name="ChangeListManager">
     <list default="true" id="c8f42924-1cd2-4b1c-bcff-602a3368bb16" name="Default Changelist" comment="">
-      <change beforePath="$PROJECT_DIR$/.idea/misc.xml" beforeDir="false" afterPath="$PROJECT_DIR$/.idea/misc.xml" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/.idea/rust.iml" beforeDir="false" afterPath="$PROJECT_DIR$/.idea/rust.iml" afterDir="false" />
       <change beforePath="$PROJECT_DIR$/.idea/workspace.xml" beforeDir="false" afterPath="$PROJECT_DIR$/.idea/workspace.xml" afterDir="false" />
     </list>
     <option name="EXCLUDED_CONVERTED_TO_IGNORED" value="true" />
@@ -15,10 +13,14 @@
   <component name="FileEditorManager">
     <leaf>
       <file pinned="false" current-in-tab="true">
-        <entry file="file://$PROJECT_DIR$/closures/src/main.rs">
+        <entry file="file://$PROJECT_DIR$/closures/src/lib.rs">
           <provider selected="true" editor-type-id="text-editor">
-            <state relative-caret-position="1136">
-              <caret line="71" lean-forward="true" selection-start-line="71" selection-end-line="71" />
+            <state relative-caret-position="1165">
+              <caret line="74" column="1" selection-start-line="74" selection-start-column="1" selection-end-line="74" selection-end-column="1" />
+              <folding>
+                <element signature="e#437#438#0" expanded="true" />
+                <element signature="e#472#473#0" expanded="true" />
+              </folding>
             </state>
           </provider>
         </entry>
@@ -82,6 +84,7 @@
         <option value="$PROJECT_DIR$/minigrep/src/lib.rs" />
         <option value="$PROJECT_DIR$/minigrep/src/main.rs" />
         <option value="$PROJECT_DIR$/closures/src/main.rs" />
+        <option value="$PROJECT_DIR$/closures/src/lib.rs" />
       </list>
     </option>
   </component>
@@ -112,6 +115,17 @@
               <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="closures" type="9f88c78c:ScopeViewTreeModel$FileNode" />
+            </path>
+            <path>
+              <item name="rust" type="3d21c010:ScopeViewTreeModel$ProjectNode" />
+              <item name="rust" type="2674bda8:ScopeViewTreeModel$GroupNode" />
+              <item name="closures" type="9f88c78c:ScopeViewTreeModel$FileNode" />
+              <item name="src" type="9f88c78c:ScopeViewTreeModel$FileNode" />
+            </path>
           </expand>
           <select />
         </subPane>
@@ -143,7 +157,7 @@
   </component>
   <component name="PropertiesComponent">
     <property name="JavaScriptWeakerCompletionTypeGuess" value="true" />
-    <property name="com.android.tools.idea.instantapp.provision.ProvisionBeforeRunTaskProvider.myTimeStamp" value="1549047706729" />
+    <property name="com.android.tools.idea.instantapp.provision.ProvisionBeforeRunTaskProvider.myTimeStamp" value="1549055111185" />
     <property name="javascript.nodejs.core.library.configured.version" value="7.1.0" />
     <property name="js.eslint.eslintPackage" value="$USER_HOME$/.yarn-config/global/node_modules/.bin/eslint" />
     <property name="js.eslint.nodeInterpreter" value="project" />
@@ -171,7 +185,7 @@
       </list>
     </option>
   </component>
-  <component name="RunManager" selected="Cargo Command.Run closures">
+  <component name="RunManager" selected="Cargo Command.Test using_other_iterator_trait_methods">
     <configuration name="&lt;template&gt;" type="TestNG" default="true" selected="false">
       <option name="MAIN_CLASS_NAME" />
       <option name="VM_PARAMETERS" value="-ea" />
@@ -200,6 +214,26 @@
       <envs />
       <method v="2" />
     </configuration>
+    <configuration name="Test calling_next_directly" type="CargoCommandRunConfiguration" factoryName="Cargo Command" temporary="true">
+      <option name="channel" value="DEFAULT" />
+      <option name="command" value="test --package closures --lib calling_next_directly -- --exact" />
+      <option name="allFeatures" value="false" />
+      <option name="nocapture" value="false" />
+      <option name="backtrace" value="SHORT" />
+      <option name="workingDirectory" value="file://$PROJECT_DIR$/closures" />
+      <envs />
+      <method v="2" />
+    </configuration>
+    <configuration name="Test filters_by_size" type="CargoCommandRunConfiguration" factoryName="Cargo Command" temporary="true">
+      <option name="channel" value="DEFAULT" />
+      <option name="command" value="test --package closures --lib filters_by_size -- --exact" />
+      <option name="allFeatures" value="false" />
+      <option name="nocapture" value="false" />
+      <option name="backtrace" value="SHORT" />
+      <option name="workingDirectory" value="file://$PROJECT_DIR$/closures" />
+      <envs />
+      <method v="2" />
+    </configuration>
     <configuration name="Test lib::tests" type="CargoCommandRunConfiguration" factoryName="Cargo Command" temporary="true">
       <option name="channel" value="DEFAULT" />
       <option name="command" value="test --package adder --lib tests" />
@@ -210,33 +244,13 @@
       <envs />
       <method v="2" />
     </configuration>
-    <configuration name="Test tests::greater_than_100" type="CargoCommandRunConfiguration" factoryName="Cargo Command" temporary="true">
+    <configuration name="Test using_other_iterator_trait_methods" type="CargoCommandRunConfiguration" factoryName="Cargo Command" temporary="true">
       <option name="channel" value="DEFAULT" />
-      <option name="command" value="test --package adder --lib tests::greater_than_100 -- --exact" />
+      <option name="command" value="test --package closures --lib using_other_iterator_trait_methods -- --exact" />
       <option name="allFeatures" value="false" />
       <option name="nocapture" value="false" />
       <option name="backtrace" value="SHORT" />
-      <option name="workingDirectory" value="file://$PROJECT_DIR$/adder" />
-      <envs />
-      <method v="2" />
-    </configuration>
-    <configuration name="Test tests::greeting_contains_name" type="CargoCommandRunConfiguration" factoryName="Cargo Command" temporary="true">
-      <option name="channel" value="DEFAULT" />
-      <option name="command" value="test --package adder --lib tests::greeting_contains_name -- --exact" />
-      <option name="allFeatures" value="false" />
-      <option name="nocapture" value="false" />
-      <option name="backtrace" value="SHORT" />
-      <option name="workingDirectory" value="file://$PROJECT_DIR$/adder" />
-      <envs />
-      <method v="2" />
-    </configuration>
-    <configuration name="Test tests::it_works" type="CargoCommandRunConfiguration" factoryName="Cargo Command" temporary="true">
-      <option name="channel" value="DEFAULT" />
-      <option name="command" value="test --package adder --lib tests::it_works -- --exact" />
-      <option name="allFeatures" value="false" />
-      <option name="nocapture" value="false" />
-      <option name="backtrace" value="SHORT" />
-      <option name="workingDirectory" value="file://$PROJECT_DIR$/adder" />
+      <option name="workingDirectory" value="file://$PROJECT_DIR$/closures" />
       <envs />
       <method v="2" />
     </configuration>
@@ -249,11 +263,11 @@
     </configuration>
     <recent_temporary>
       <list>
+        <item itemvalue="Cargo Command.Test using_other_iterator_trait_methods" />
+        <item itemvalue="Cargo Command.Test calling_next_directly" />
+        <item itemvalue="Cargo Command.Test filters_by_size" />
         <item itemvalue="Cargo Command.Run closures" />
         <item itemvalue="Cargo Command.Test lib::tests" />
-        <item itemvalue="Cargo Command.Test tests::greater_than_100" />
-        <item itemvalue="Cargo Command.Test tests::greeting_contains_name" />
-        <item itemvalue="Cargo Command.Test tests::it_works" />
       </list>
     </recent_temporary>
   </component>
@@ -276,13 +290,19 @@
     <history-entry file="Test_lib__tests - 2019.01.31 at 14h 59m 57s.xml">
       <configuration name="Test lib::tests" configurationId="CargoCommandRunConfiguration" />
     </history-entry>
+    <history-entry file="Test_filters_by_size - 2019.02.01 at 15h 58m 53s.xml">
+      <configuration name="Test filters_by_size" configurationId="CargoCommandRunConfiguration" />
+    </history-entry>
+    <history-entry file="Test_calling_next_directly - 2019.02.01 at 16h 04m 05s.xml">
+      <configuration name="Test calling_next_directly" configurationId="CargoCommandRunConfiguration" />
+    </history-entry>
   </component>
   <component name="ToolWindowManager">
     <frame x="1920" y="-438" width="1080" height="1897" extended-state="6" />
     <editor active="true" />
     <layout>
-      <window_info active="true" content_ui="combo" id="Project" order="0" sideWeight="0.49692565" visible="true" weight="0.2540132" />
-      <window_info id="Structure" order="1" sideWeight="0.50307435" side_tool="true" visible="true" weight="0.2540132" />
+      <window_info content_ui="combo" id="Project" order="0" sideWeight="0.49642006" visible="true" weight="0.2540132" />
+      <window_info id="Structure" order="1" sideWeight="0.50358" side_tool="true" visible="true" weight="0.2540132" />
       <window_info id="Image Layers" order="2" />
       <window_info id="Designer" order="3" />
       <window_info id="Capture Tool" order="4" />
@@ -290,7 +310,7 @@
       <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 anchor="bottom" id="Run" order="2" weight="0.29737285" />
+      <window_info active="true" anchor="bottom" id="Run" order="2" visible="true" weight="0.29737285" />
       <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" />
@@ -603,8 +623,19 @@
     </entry>
     <entry file="file://$PROJECT_DIR$/closures/src/main.rs">
       <provider selected="true" editor-type-id="text-editor">
-        <state relative-caret-position="1136">
-          <caret line="71" lean-forward="true" selection-start-line="71" selection-end-line="71" />
+        <state relative-caret-position="1120">
+          <caret line="70" column="1" selection-end-line="71" />
+        </state>
+      </provider>
+    </entry>
+    <entry file="file://$PROJECT_DIR$/closures/src/lib.rs">
+      <provider selected="true" editor-type-id="text-editor">
+        <state relative-caret-position="1165">
+          <caret line="74" column="1" selection-start-line="74" selection-start-column="1" selection-end-line="74" selection-end-column="1" />
+          <folding>
+            <element signature="e#437#438#0" expanded="true" />
+            <element signature="e#472#473#0" expanded="true" />
+          </folding>
         </state>
       </provider>
     </entry>
diff --git a/closures/src/lib.rs b/closures/src/lib.rs
new file mode 100644
index 0000000..80a11e5
--- /dev/null
+++ b/closures/src/lib.rs
@@ -0,0 +1,75 @@
+#[derive(PartialEq, Debug)]
+struct Shoe {
+    size: u32,
+    style: String,
+}
+
+fn shoes_in_my_size(shoes: Vec<Shoe>, shoe_size: u32) -> Vec<Shoe> {
+    shoes.into_iter() // into_iter takes ownership of the original vector
+        .filter(|s| s.size == shoe_size) // Filter is lazy!
+        .collect() // Tell the iterator to run, and give me a collection
+}
+
+struct Counter {
+    count: u32,
+}
+
+impl Counter {
+    fn new() -> Counter {
+        Counter { count: 0 }
+    }
+}
+
+impl Iterator for Counter {
+    type Item = u32;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.count += 1;
+
+        if self.count < 6 {
+            Some(self.count)
+        } else {
+            None
+        }
+    }
+}
+
+#[test]
+fn filters_by_size() {
+    let shoes = vec![
+        Shoe { size: 10, style: String::from("sneaker") },
+        Shoe { size: 13, style: String::from("sandal") },
+        Shoe { size: 10, style: String::from("boot") },
+    ];
+
+    let in_my_size = shoes_in_my_size(shoes, 10);
+
+    assert_eq!(
+        in_my_size,
+        vec![
+            Shoe { size: 10, style: String::from("sneaker") },
+            Shoe { size: 10, style: String::from("boot") },
+        ]
+    );
+}
+
+#[test]
+fn calling_next_directly() {
+    let mut counter = Counter::new();
+
+    assert_eq!(counter.next(), Some(1));
+    assert_eq!(counter.next(), Some(2));
+    assert_eq!(counter.next(), Some(3));
+    assert_eq!(counter.next(), Some(4));
+    assert_eq!(counter.next(), Some(5));
+    assert_eq!(counter.next(), None);
+}
+
+#[test]
+fn using_other_iterator_trait_methods() {
+    let sum: u32 = Counter::new().zip(Counter::new().skip(1))
+        .map(|(a, b)| a * b)
+        .filter(|x| x % 3 == 0)
+        .sum();
+    assert_eq!(18, sum);
+}