Clarify the binary search tree exercise

src/SUMMARY.md

@@ -158,7 +158,7 @@ SPDX-License-Identifier: CC-BY-4.0
   - [`Box<T>`](smart-pointers/box.md)
   - [`Rc`](smart-pointers/rc.md)
   - [Owned Trait Objects](smart-pointers/trait-objects.md)
-  - [Exercise: Binary Tree](smart-pointers/exercise.md)
+  - [Exercise: Binary Search Tree](smart-pointers/exercise.md)
     - [Solution](smart-pointers/solution.md)
 
 # Day 3: Afternoon

src/smart-pointers/exercise.md

@@ -7,13 +7,15 @@ Copyright 2023 Google LLC
 SPDX-License-Identifier: CC-BY-4.0
 -->
 
-# Exercise: Binary Tree
+# Exercise: Binary Search Tree
 
-A binary tree is a tree-type data structure where every node has two children
-(left and right). We will create a tree where each node stores a value. For a
-given node N, all nodes in a N's left subtree contain smaller values, and all
-nodes in N's right subtree will contain larger values. A given value should only
-be stored in the tree once, i.e. no duplicate nodes.
+A _binary tree_ is a tree-type data structure where each node has at most two
+children (left and right). In this exercise, we will implement a _binary search
+tree_, which maintains an ordering invariant: for every node, all values in its
+left subtree are smaller than its value, and all values in its right subtree are
+larger. The `insert` and `has` methods rely on this ordering to decide which
+subtree to visit. A given value should only be stored in the tree once, i.e. no
+duplicate nodes.
 
 Implement the following types, so that the given tests pass.
 

src/smart-pointers/exercise.rs

@@ -18,7 +18,7 @@
 use std::cmp::Ordering;
 
 // ANCHOR: types
-/// A node in the binary tree.
+/// A node in the binary search tree.
 #[derive(Debug)]
 struct Node<T: Ord> {
     value: T,
@@ -30,7 +30,7 @@ struct Node<T: Ord> {
 #[derive(Debug)]
 struct Subtree<T: Ord>(Option<Box<Node<T>>>);
 
-/// A container storing a set of values, using a binary tree.
+/// A container storing a set of values, using a binary search tree.
 ///
 /// If the same value is added multiple times, it is only stored once.
 #[derive(Debug)]