diff --git a/sub_crates/backend/src/buffer.rs b/sub_crates/backend/src/buffer.rs
index e489f83..496ddf1 100644
--- a/sub_crates/backend/src/buffer.rs
+++ b/sub_crates/backend/src/buffer.rs
@@ -5,6 +5,17 @@ use ropey::Rope;
 pub struct Buffer {
     pub is_dirty: bool, // Is this buffer currently out of sync with disk.
     pub text: Rope,     // The actual text content.
+
+    // Sets of marked ranges to be used for various purposes.
+    //
+    // Each individual mark consists of a head and a tail.  The ordering of the
+    // head and tail is unspecified: either the head or the tail may come first
+    // in the text.  Both head and tail are specified in absolute indices.
+    //
+    // Within sets, marks cannot overlap or abutt, and they must be ordered.
+    // Overlapping or abutting marks will be merged, and out-of-order marks
+    // will be removed.
+    pub mark_sets: Vec<Vec<(usize, usize)>>,
 }
 
 impl Buffer {
@@ -12,16 +23,43 @@ impl Buffer {
         Buffer {
             is_dirty: false,
             text: text,
+            mark_sets: Vec::new(),
         }
     }
 
-    pub fn insert(&mut self, text: &str, char_idx: usize) {
-        self.text.insert(char_idx, text);
+    // Replaces the given range of chars with the given text.
+    pub fn edit(&mut self, char_idx_range: (usize, usize), text: &str) {
         self.is_dirty = true;
+
+        // Get the range, properly ordered.
+        let (start, end) = if char_idx_range.0 < char_idx_range.1 {
+            (char_idx_range.0, char_idx_range.1)
+        } else {
+            (char_idx_range.1, char_idx_range.0)
+        };
+
+        // Do removal if needed.
+        if start != end {
+            self.text.remove(start..end);
+        }
+
+        // Do insertion if needed.
+        if !text.is_empty() {
+            self.text.insert(start, text);
+        }
     }
 
-    pub fn remove(&mut self, char_start: usize, char_end: usize) {
-        self.text.remove(char_start..char_end);
-        self.is_dirty = true;
+    /// Creates a new empty mark set, and returns the set index.
+    pub fn add_mark_set(&mut self) -> usize {
+        self.mark_sets.push(vec![(0, 0)]);
+        return self.mark_sets.len() - 1;
+    }
+
+    pub fn insert_new_mark(&mut self, _set_idx: usize, _mark: (usize, usize)) {
+        todo!()
+    }
+
+    pub fn clear_mark_set(&mut self, set_idx: usize) {
+        self.mark_sets[set_idx].clear();
     }
 }