diff --git a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/Cargo.toml b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/Cargo.toml
new file mode 100644
index 0000000..56f4002
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "workout_generator"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/lib.rs b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/lib.rs
new file mode 100644
index 0000000..e3c76b4
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/lib.rs
@@ -0,0 +1 @@
+pub mod workout;
diff --git a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/main.rs b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/main.rs
new file mode 100644
index 0000000..97804d3
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/main.rs
@@ -0,0 +1,8 @@
+use workout_generator::workout;
+
+fn main() {
+    let simulated_user_specified_value = 10;
+    let simulated_random_number = 7;
+
+    workout::generate_workout(simulated_user_specified_value, simulated_random_number);
+}
diff --git a/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs
new file mode 100644
index 0000000..5a3f598
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/01-abstract-behavior/workout_generator/src/workout.rs
@@ -0,0 +1,66 @@
+use std::{thread, time::Duration};
+
+fn simulated_expensive_calculation(intensity: u32) -> u32 {
+    println!("calculating slowly...");
+    thread::sleep(Duration::from_secs(2)); // like timeout, timer
+    intensity
+}
+
+// Iteration #3: Define a closure, and store this definition in a variable so that it can be called later.
+// This reintroduces the problem of the expensive calculation bein executed multiple times.
+// The solution will be presented in the next crate in this series.
+pub fn generate_workout(intensity: u32, random_number: u32) {
+    let expensive_closure = |num| simulated_expensive_calculation(num);
+
+    if intensity < 25 {
+        println!("Today, do {} pushups!", expensive_closure(intensity));
+        println!("Next, do {} situps!", expensive_closure(intensity));
+    } else {
+        if random_number == 3 {
+            println!("Take a break today! Remeber to stay hydrated!");
+        } else {
+            println!("Today, run for {} minutes!", expensive_closure(intensity));
+        }
+    }
+}
+
+// Iteration #2 - refactor by storing the result of duplicated function calls in a variable.
+// Still not ideal as expensive result gets run at least once, although it's not
+// needed by the `random_number == 3` `if` block.
+// pub fn generate_workout(intensity: u32, random_number: u32) {
+//     let expensive_result = simulated_expensive_calculation(intensity);
+
+//     if intensity < 25 {
+//         println!("Today, do {} pushups!", expensive_result);
+//         println!("Next, do {} situps!", expensive_result);
+//     } else {
+//         if random_number == 3 {
+//             println!("Take a break today! Remeber to stay hydrated!");
+//         } else {
+//             println!("Today, run for {} minutes!", expensive_result);
+//         }
+//     }
+// }
+
+// Iteration #1 - expensive calculation called multiple times instead of once.
+// pub fn generate_workout(intensity: u32, random_number: u32) {
+//     if intensity < 25 {
+//         println!(
+//             "Today, do {} pushups!",
+//             simulated_expensive_calculation(intensity)
+//         );
+//         println!(
+//             "Next, do {} situps!",
+//             simulated_expensive_calculation(intensity)
+//         );
+//     } else {
+//         if random_number == 3 {
+//             println!("Take a break today! Remeber to stay hydrated!");
+//         } else {
+//             println!(
+//                 "Today, run for {} minutes!",
+//                 simulated_expensive_calculation(intensity)
+//             );
+//         }
+//     }
+// }
diff --git a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/Cargo.toml b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/Cargo.toml
new file mode 100644
index 0000000..56f4002
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "workout_generator"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs
new file mode 100644
index 0000000..63cfa33
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/cacher.rs
@@ -0,0 +1,47 @@
+// `Cacher` struct:
+// - holds a closure in `calculation` field
+// - holds an optional result, `Option<u32>`, in the `value` field
+pub struct Cacher<T>
+where
+    T: Fn(u32) -> u32,
+{
+    //   - `calculation` field is of the generic type `T`
+    //   - trait bounds on `T` specify that it's a closure by using the `Fn` trait
+    //   - closure stored on `calculation` field must have on `u32` parameter and
+    //     must return a `u32`
+    calculation: T,
+
+    //   The following logic is defined in `impl<T> Cacher<T>`:
+    //   - before the closure is executed, `value` will be `None`
+    //   - when code using a `Cacher` askd for the result of the closure, the `Cacher`
+    //     will execute the closure and store the result within a `Some` variant in the
+    //     `value` field
+    //   - if the code asks for the result of the clousure again, instead of executing the
+    //     closure again, the `Cacher` will return the result held in the `Some` variant
+    value: Option<u32>,
+}
+
+impl<T> Cacher<T>
+where
+    T: Fn(u32) -> u32,
+{
+    // takes a generic parameter `T`, which is defined as having the same trait bound
+    // as the `Cacher` struct
+    pub fn new(calculation: T) -> Cacher<T> {
+        Cacher {
+            calculation,
+            value: None,
+        }
+    }
+
+    pub fn value(&mut self, arg: u32) -> u32 {
+        match self.value {
+            Some(v) => v,
+            None => {
+                let v = (self.calculation)(arg);
+                self.value = Some(v);
+                v
+            }
+        }
+    }
+}
diff --git a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/lib.rs b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/lib.rs
new file mode 100644
index 0000000..f09b820
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/lib.rs
@@ -0,0 +1,2 @@
+pub mod cacher;
+pub mod workout;
diff --git a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/main.rs b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/main.rs
new file mode 100644
index 0000000..f077a05
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/main.rs
@@ -0,0 +1,20 @@
+use workout_generator::workout;
+
+fn main() {
+    // calculating slowly...
+    // Today, do 10 pushups!
+    // Next, do 10 situps!
+    let simulated_user_specified_value = 10;
+    let simulated_random_number = 7;
+
+    // calculating slowly...
+    // Today, run for 100 minutes!
+    // let simulated_user_specified_value = 100;
+    // let simulated_random_number = 7;
+
+    // Take a break today! Remeber to stay hydrated!
+    // let simulated_user_specified_value = 100;
+    // let simulated_random_number = 3;
+
+    workout::generate_workout(simulated_user_specified_value, simulated_random_number);
+}
diff --git a/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/workout.rs b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/workout.rs
new file mode 100644
index 0000000..56bd51a
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/02-memoization-with-struct/workout_generator/src/workout.rs
@@ -0,0 +1,25 @@
+use crate::cacher::Cacher;
+use std::{thread, time::Duration};
+
+// Iteration #: Using `Cacher` struct
+pub fn generate_workout(intensity: u32, random_number: u32) {
+    let mut expensive_result = Cacher::new(|num| {
+        println!("calculating slowly...");
+        thread::sleep(Duration::from_secs(2));
+        num
+    });
+
+    if intensity < 25 {
+        println!("Today, do {} pushups!", expensive_result.value(intensity));
+        println!("Next, do {} situps!", expensive_result.value(intensity));
+    } else {
+        if random_number == 3 {
+            println!("Take a break today! Remeber to stay hydrated!");
+        } else {
+            println!(
+                "Today, run for {} minutes!",
+                expensive_result.value(intensity)
+            );
+        }
+    }
+}
diff --git a/13-functional-language-features/13-1-closures/README.md b/13-functional-language-features/13-1-closures/README.md
new file mode 100644
index 0000000..28154e1
--- /dev/null
+++ b/13-functional-language-features/13-1-closures/README.md
@@ -0,0 +1,19 @@
+# 13.1. Closures: Anonymous functions that can capture their environment
+
+Closures:
+
+- can be saved in a variable
+- can be passed as arguments to functions
+- can be created in one place, and then later called to evaluate it in a different context
+- can capture values from the scope in which they're defined (unlike functions)
+- allow for code reuse
+- allow for behavior customisation
+- need to be called at least once for the compiler to infer the types of their parameters & return value, if no arguments from the outer scope are passed into the definition
+
+## Sample code
+
+[Naively call expensive function multiple times as needed](./01-abstract-behavior/workout_generator/src/workout.rs)
+
+[Lazy evaluation / lazily evaluate / memoization / memo / cache / caching implementation with a `Cacher` struct typed with a `Fn` trait bound that closures must match](./02-memoization-with-struct/workout_generator/src/)
+
+[Capturing the environment with Closures](TODO)
diff --git a/13-functional-language-features/13-2-iterators/README.md b/13-functional-language-features/13-2-iterators/README.md
new file mode 100644
index 0000000..802f546
--- /dev/null
+++ b/13-functional-language-features/13-2-iterators/README.md
@@ -0,0 +1,110 @@
+# Processing a series of items with iterators
+
+- iterator pattern is code that performs some task on a sequence of items in turn
+- an iterator is responsible for:
+  - the logic of iterating over each item
+  - determining when a sequence is finished
+- an iterator is lazy:
+  - it has no effect until...
+  - ...the code calls methods that consume the iterator to use it up
+
+```rust
+let v1 = vec![1, 2, 3]; // vector `v1`
+
+// - call `iter` method defined on `Vec<T>`
+//   -> creates iterator `v1_iter` over items in vector `v1`
+// - at this point, the code does not do anything useful;
+//   - remember that an iterator is lazy
+let v1_iter = v1.iter();
+
+// use `v1_iter` with a `for` loop to run code on each item on every iteration
+for val in v1_iter {
+  println!("Got: {}", val);
+}
+```
+
+## The `Iterator` trait and the `next` method
+
+Iterators implement a trait named `Iterator` that is defined in the standard library:
+
+```rust
+pub trait Iterator {
+  // new syntax (A): `type Item`
+  type Item;
+
+  // new syntax (A): `Self::Item`
+  fn next(&mut self) -> Option<Self::Item>;
+
+  // methods with default implementations elided
+}
+```
+
+**`new syntax (A)`** defines an [associated type](../../19-advanced-features/19-2-advanced-traits/README.md/#specifying-placeholder-types-in-trait-definitions-with-associated-types) with this trait:
+
+- implementing the `Iterator` trait requires an `Item` type to be defined
+- the `Item` type is used in the return type of the `next` method
+- the `Item` type will be the type returned from the iterator
+
+Implementors of the `Iterator` trait must define one method, the `next` method:
+
+- which returns one item of the iterator at a time wrapped in `Some`,
+- and, when the iteration is over, returns `None`
+
+The `next` method can be called on iterators directly:
+
+```rust
+#[test]
+fn iterator_demonstration() {
+  let v1 = vec![1, 2, 3];
+
+  // Note (A)
+  // v1_iter must be mutable, otherwise error on `v1_iter.next()`:
+  //   cannot borrow `v1_iter` as mutable, as it is not declared as mutable
+  let mut v1_iter = v1.iter();
+
+  // Note (B)
+  // `Some(&1)`, otherwise error:
+  //   expected `&{integer}`, found integer
+  assert_eq!(v1_iter.next(), Some(&1));
+  assert_eq!(v1_iter.next(), Some(&2));
+  assert_eq!(v1_iter.next(), Some(&3));
+  assert_eq!(v1_iter.next(), None);
+
+  // Note (C) `into_iter` takes ownership of v1
+  let v1 = vec![1, 2, 3];
+  let mut v1_iter = v1.into_iter();
+  //        Error (E1) ----------- `v1` moved due to this method call
+  // println!("{:?}", v1); // Error (E1) will occur
+  //       Error (E1) ^^ value borrowed here after move
+
+  assert_eq!(v1_iter.next(), Some(1));
+  assert_eq!(v1_iter.next(), Some(2));
+  assert_eq!(v1_iter.next(), Some(3));
+  assert_eq!(v1_iter.next(), None);
+
+  // Note (D) `iter_mut`
+  let mut v1 = vec![1, 2, 3];
+  let mut v1_iter = v1.iter_mut();
+  assert_eq!(v1_iter.next(), Some(&mut 1));
+  assert_eq!(v1_iter.next(), Some(&mut 2));
+  assert_eq!(v1_iter.next(), Some(&mut 3));
+  assert_eq!(v1_iter.next(), None);
+}
+```
+
+**Note (A)**: Iterator `v1_iter` needs to be made mutable:
+
+- calling the `next` method changes the internal state used by the iterator:
+  - the internal state is used to keep track of where the iterator is in the sequence
+- calling `next`:
+  - consumes, or uses up, the iterator
+  - eats up an iterm from the iterator
+- `v1_iter` needn't be made mutable when used in a `for` loop because:
+  - the loop takes ownership of `v1_iter` and makes it mutable behind the scenes
+
+**Note (B)**: Values obtained from calls to `next`:
+
+- are immutable references to the values in the vector `v1`
+- the `iter` method in `v1.iter()` produces an iterator over immutable references:
+  - **Note (C)**: `into_iter` - creates an iterator that takes ownership of `v1` and returns owned values
+  - **Note (D)**: `iter_mut` - creates an iterator over mutable references
diff --git a/13-functional-language-features/13-2-iterators/iterators/Cargo.toml b/13-functional-language-features/13-2-iterators/iterators/Cargo.toml
new file mode 100644
index 0000000..069541a
--- /dev/null
+++ b/13-functional-language-features/13-2-iterators/iterators/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "iterators"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/13-functional-language-features/13-2-iterators/iterators/src/main.rs b/13-functional-language-features/13-2-iterators/iterators/src/main.rs
new file mode 100644
index 0000000..b46182b
--- /dev/null
+++ b/13-functional-language-features/13-2-iterators/iterators/src/main.rs
@@ -0,0 +1,23 @@
+fn main() {
+    let v = vec![1, 2, 3];
+    let mut v_iter = v.iter();
+    assert_eq!(v_iter.next(), Some(&1));
+    assert_eq!(v_iter.next(), Some(&2));
+    assert_eq!(v_iter.next(), Some(&3));
+    assert_eq!(v_iter.next(), None);
+
+    let v1 = vec![1, 2, 3];
+    let mut v_iter = v1.into_iter();
+    println!("{:?}", v1);
+    assert_eq!(v_iter.next(), Some(1));
+    assert_eq!(v_iter.next(), Some(2));
+    assert_eq!(v_iter.next(), Some(3));
+    assert_eq!(v_iter.next(), None);
+
+    let mut v = vec![1, 2, 3];
+    let mut v_iter = v.iter_mut();
+    assert_eq!(v_iter.next(), Some(&mut 1));
+    assert_eq!(v_iter.next(), Some(&mut 2));
+    assert_eq!(v_iter.next(), Some(&mut 3));
+    assert_eq!(v_iter.next(), None);
+}
diff --git a/13-functional-language-features/README.md b/13-functional-language-features/README.md
new file mode 100644
index 0000000..f719f39
--- /dev/null
+++ b/13-functional-language-features/README.md
@@ -0,0 +1 @@
+# 13. Functional language features: iterators and closures
diff --git a/19-advanced-features/19-2-advanced-traits/README.md b/19-advanced-features/19-2-advanced-traits/README.md
new file mode 100644
index 0000000..1cee064
--- /dev/null
+++ b/19-advanced-features/19-2-advanced-traits/README.md
@@ -0,0 +1,7 @@
+# Advanced traits
+
+## Specifying placeholder types in trait definitions with associated types
+
+Referenced by:
+
+- [The `Iterator` trait and the `next` method](../../13-functional-language-features/13-2-iterators/README.md/#the-iterator-trait-and-the-next-method)
diff --git a/19-advanced-features/README.md b/19-advanced-features/README.md
new file mode 100644
index 0000000..7c935bd
--- /dev/null
+++ b/19-advanced-features/README.md
@@ -0,0 +1 @@
+# Advanced features