feat: get first or last element of OneToManyUniqueBTreeMap

src/one_to_many.rs

@@ -42,6 +42,14 @@ where
         self.right_to_left.get(right)
     }
 
+    pub fn first_left_and_rights(&self) -> Option<(&Left, &BTreeSet<Right>)> {
+        self.left_to_rights.first_key_value()
+    }
+
+    pub fn last_left_and_rights(&self) -> Option<(&Left, &BTreeSet<Right>)> {
+        self.left_to_rights.last_key_value()
+    }
+
     pub fn remove_left(&mut self, left: &Left) -> Option<(Left, BTreeSet<Right>)> {
         let (left, rights) = self.left_to_rights.remove_entry(left)?;