diff --git a/sub_crates/backend/src/buffer.rs b/sub_crates/backend/src/buffer.rs
index e383165..80050d0 100644
--- a/sub_crates/backend/src/buffer.rs
+++ b/sub_crates/backend/src/buffer.rs
@@ -57,7 +57,7 @@ impl Buffer {
         // Update mark sets.
         let post_len = text.chars().count();
         for mark_set in self.mark_sets.iter_mut() {
-            for mark in mark_set.marks.iter_mut() {
+            for mark in mark_set.iter_mut() {
                 *mark = mark.edit((start, end), post_len);
             }
 
@@ -88,7 +88,7 @@ impl Buffer {
 
             // Update mark sets.
             for mark_set in self.mark_sets.iter_mut() {
-                for mark in mark_set.marks.iter_mut() {
+                for mark in mark_set.iter_mut() {
                     *mark = mark.edit((start, end), post_len);
                 }
 
@@ -124,7 +124,7 @@ impl Buffer {
 
             // Update mark sets.
             for mark_set in self.mark_sets.iter_mut() {
-                for mark in mark_set.marks.iter_mut() {
+                for mark in mark_set.iter_mut() {
                     *mark = mark.edit((start, end), post_len);
                 }
 
diff --git a/sub_crates/backend/src/marks.rs b/sub_crates/backend/src/marks.rs
index 2c2bed5..383769d 100644
--- a/sub_crates/backend/src/marks.rs
+++ b/sub_crates/backend/src/marks.rs
@@ -208,4 +208,26 @@ impl MarkSet {
 
         self.marks.truncate(i1 + 1);
     }
+
+    pub fn iter(&self) -> std::slice::Iter<Mark> {
+        self.marks.iter()
+    }
+
+    pub fn iter_mut(&mut self) -> std::slice::IterMut<Mark> {
+        self.marks.iter_mut()
+    }
+}
+
+impl std::ops::Index<usize> for MarkSet {
+    type Output = Mark;
+
+    fn index(&self, index: usize) -> &Mark {
+        &self.marks[index]
+    }
+}
+
+impl std::ops::IndexMut<usize> for MarkSet {
+    fn index_mut(&mut self, index: usize) -> &mut Mark {
+        &mut self.marks[index]
+    }
 }