Fill in a BTreeSet::entry example

alloc/src/collections/btree/set.rs

@@ -993,7 +993,37 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     ///
     /// # Examples
     ///
-    /// TODO
+    /// ```
+    /// #![feature(btree_set_entry)]
+    ///
+    /// use std::collections::BTreeSet;
+    /// use std::collections::btree_set::Entry::*;
+    ///
+    /// let mut singles = BTreeSet::new();
+    /// let mut dupes = BTreeSet::new();
+    ///
+    /// for ch in "a short treatise on fungi".chars() {
+    ///     if let Vacant(dupe_entry) = dupes.entry(ch) {
+    ///         // We haven't already seen a duplicate, so
+    ///         // check if we've at least seen it once.
+    ///         match singles.entry(ch) {
+    ///             Vacant(single_entry) => {
+    ///                 // We found a new character for the first time.
+    ///                 single_entry.insert()
+    ///             }
+    ///             Occupied(single_entry) => {
+    ///                 // We've already seen this once, "move" it to dupes.
+    ///                 single_entry.remove();
+    ///                 dupe_entry.insert();
+    ///             }
+    ///         }
+    ///     }
+    /// }
+    ///
+    /// assert!(!singles.contains(&'t') && dupes.contains(&'t'));
+    /// assert!(singles.contains(&'u') && !dupes.contains(&'u'));
+    /// assert!(!singles.contains(&'v') && !dupes.contains(&'v'));
+    /// ```
     #[inline]
     #[unstable(feature = "btree_set_entry", issue = "133549")]
     pub fn entry(&mut self, value: T) -> Entry<'_, T, A>