diff --git a/src/impls/storage.rs b/src/impls/storage.rs
index c89d947..0a1b5b7 100644
--- a/src/impls/storage.rs
+++ b/src/impls/storage.rs
@@ -191,7 +191,10 @@ impl<T> Storage<T> for Doubling<T> {
     #[inline]
     fn heap_size<F: FnMut(usize, usize)>(&self, mut callback: F) {
         let size_of_usize = std::mem::size_of::<usize>();
-        callback(self.offsets.len() * size_of_usize, self.offsets.capacity() * size_of_usize);
+        callback(
+            self.offsets.len() * size_of_usize,
+            self.offsets.capacity() * size_of_usize,
+        );
         let size_of_t = std::mem::size_of::<T>();
         for inner in &self.inner {
             callback(inner.len() * size_of_t, inner.capacity() * size_of_t);
diff --git a/src/lib.rs b/src/lib.rs
index fbd7bbc..411d0c3 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -15,6 +15,7 @@ pub use impls::option::OptionRegion;
 pub use impls::result::ResultRegion;
 pub use impls::slice::SliceRegion;
 pub use impls::slice_copy::OwnedRegion;
+pub use impls::storage::Storage;
 pub use impls::string::StringRegion;
 
 /// An index into a region. Automatically implemented for relevant types.
@@ -145,51 +146,47 @@ impl<'a, T: ToOwned + ?Sized> IntoOwned<'a> for &'a T {
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 #[cfg_attr(
     feature = "serde",
-    serde(
-        bound = "R: Serialize + for<'a> Deserialize<'a>, R::Index: Serialize + for<'a> Deserialize<'a>"
-    )
+    serde(bound = "
+            R: Serialize + for<'a> Deserialize<'a>,
+            R::Index: Serialize + for<'a> Deserialize<'a>,
+            S: Serialize + for<'a> Deserialize<'a>,
+            ")
 )]
-pub struct FlatStack<R: Region> {
+pub struct FlatStack<R: Region, S: Storage<R::Index> = Vec<<R as Region>::Index>> {
     /// The indices, which we use to lookup items in the region.
-    indices: Vec<R::Index>,
+    indices: S,
     /// A region to index into.
     region: R,
 }
 
-impl<R: Region> Default for FlatStack<R> {
+impl<R: Region, S: Storage<<R as Region>::Index>> Default for FlatStack<R, S> {
     #[inline]
     fn default() -> Self {
         Self {
-            indices: Vec::default(),
+            indices: S::default(),
             region: R::default(),
         }
     }
 }
 
-impl<R: Region> Debug for FlatStack<R>
+impl<R: Region, S: Storage<<R as Region>::Index>> Debug for FlatStack<R, S>
 where
     for<'a> R::ReadItem<'a>: Debug,
+    for<'a> &'a S: IntoIterator<Item = &'a R::Index>,
 {
     fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
         f.debug_list().entries(self.iter()).finish()
     }
 }
 
-impl<R: Region> FlatStack<R> {
-    /// Default implementation based on the preference of type `T`.
-    #[inline]
-    #[must_use]
-    pub fn default_impl<T: Containerized<Region = R>>() -> Self {
-        Self::default()
-    }
-
+impl<R: Region, S: Storage<<R as Region>::Index>> FlatStack<R, S> {
     /// Returns a flat stack that can absorb `capacity` indices without reallocation.
     ///
     /// Prefer [`Self::merge_capacity`] over this function to also pre-size the regions.
     #[must_use]
     pub fn with_capacity(capacity: usize) -> Self {
         Self {
-            indices: Vec::with_capacity(capacity),
+            indices: S::with_capacity(capacity),
             region: R::default(),
         }
     }
@@ -198,10 +195,10 @@ impl<R: Region> FlatStack<R> {
     #[must_use]
     pub fn merge_capacity<'a, I: Iterator<Item = &'a Self> + Clone + 'a>(stacks: I) -> Self
     where
-        R: 'a,
+        Self: 'a,
     {
         Self {
-            indices: Vec::with_capacity(stacks.clone().map(|s| s.indices.len()).sum()),
+            indices: S::merge_regions(stacks.clone().map(|s| &s.indices)),
             region: R::merge_regions(stacks.map(|r| &r.region)),
         }
     }
@@ -213,14 +210,14 @@ impl<R: Region> FlatStack<R> {
         R: Push<T>,
     {
         let index = self.region.push(item);
-        self.indices.push(index);
+        self.indices.extend_from_slice(&[index]);
     }
 
     /// Returns the element at the `offset` position.
     #[inline]
     #[must_use]
     pub fn get(&self, offset: usize) -> R::ReadItem<'_> {
-        self.region.index(self.indices[offset])
+        self.region.index(self.indices.index(offset, offset + 1)[0])
     }
 
     /// Returns the number of indices in the stack.
@@ -237,12 +234,6 @@ impl<R: Region> FlatStack<R> {
         self.indices.is_empty()
     }
 
-    /// Returns the total number of indices the stack can hold without reallocation.
-    #[must_use]
-    pub fn capacity(&self) -> usize {
-        self.indices.capacity()
-    }
-
     /// Reserves space to hold `additional` indices.
     #[inline]
     pub fn reserve(&mut self, additional: usize) {
@@ -274,18 +265,41 @@ impl<R: Region> FlatStack<R> {
         self.region.reserve_regions(regions);
     }
 
+    /// Heap size, size - capacity
+    #[inline]
+    pub fn heap_size<F: FnMut(usize, usize)>(&self, mut callback: F) {
+        self.region.heap_size(&mut callback);
+        self.indices.heap_size(callback);
+    }
+}
+
+impl<R, S> FlatStack<R, S>
+where
+    R: Region,
+    S: Storage<<R as Region>::Index>,
+{
     /// Iterate the items in this stack.
     #[inline]
-    pub fn iter(&self) -> Iter<'_, R> {
+    pub fn iter<'a>(&'a self) -> Iter<'a, R, <&'a S as IntoIterator>::IntoIter>
+    where
+        &'a S: IntoIterator<Item = &'a R::Index>,
+    {
         self.into_iter()
     }
+}
 
-    /// Heap size, size - capacity
+impl<R: Region> FlatStack<R> {
+    /// Default implementation based on the preference of type `T`.
     #[inline]
-    pub fn heap_size<F: FnMut(usize, usize)>(&self, mut callback: F) {
-        use crate::impls::offsets::OffsetContainer;
-        self.region.heap_size(&mut callback);
-        self.indices.heap_size(callback);
+    #[must_use]
+    pub fn default_impl<T: Containerized<Region = R>>() -> Self {
+        Self::default()
+    }
+
+    /// Returns the total number of indices the stack can hold without reallocation.
+    #[must_use]
+    pub fn capacity(&self) -> usize {
+        self.indices.capacity()
     }
 }
 
@@ -299,13 +313,16 @@ impl<T, R: Region + Push<T>> Extend<T> for FlatStack<R> {
     }
 }
 
-impl<'a, R: Region> IntoIterator for &'a FlatStack<R> {
+impl<'a, R: Region, S: Storage<<R as Region>::Index>> IntoIterator for &'a FlatStack<R, S>
+where
+    &'a S: IntoIterator<Item = &'a <R as Region>::Index>,
+{
     type Item = R::ReadItem<'a>;
-    type IntoIter = Iter<'a, R>;
+    type IntoIter = Iter<'a, R, <&'a S as IntoIterator>::IntoIter>;
 
     fn into_iter(self) -> Self::IntoIter {
         Iter {
-            inner: self.indices.iter(),
+            inner: self.indices.into_iter(),
             region: &self.region,
         }
     }
@@ -313,14 +330,22 @@ impl<'a, R: Region> IntoIterator for &'a FlatStack<R> {
 
 /// An iterator over [`FlatStack`]. The iterator yields [`Region::ReadItem`] elements, which
 /// it obtains by looking up indices.
-pub struct Iter<'a, R: Region> {
+pub struct Iter<'a, R, S>
+where
+    R: Region,
+    S: Iterator<Item = &'a <R as Region>::Index>,
+{
     /// Iterator over indices.
-    inner: std::slice::Iter<'a, R::Index>,
+    inner: S,
     /// Region to map indices to read items.
     region: &'a R,
 }
 
-impl<'a, R: Region> Iterator for Iter<'a, R> {
+impl<'a, R, S> Iterator for Iter<'a, R, S>
+where
+    R: Region,
+    S: Iterator<Item = &'a <R as Region>::Index>,
+{
     type Item = R::ReadItem<'a>;
 
     fn next(&mut self) -> Option<Self::Item> {
@@ -332,9 +357,18 @@ impl<'a, R: Region> Iterator for Iter<'a, R> {
     }
 }
 
-impl<R: Region> ExactSizeIterator for Iter<'_, R> {}
+impl<'a, R, S> ExactSizeIterator for Iter<'a, R, S>
+where
+    R: Region,
+    S: ExactSizeIterator<Item = &'a <R as Region>::Index>,
+{
+}
 
-impl<R: Region> Clone for Iter<'_, R> {
+impl<'a, R, S> Clone for Iter<'a, R, S>
+where
+    R: Region,
+    S: Iterator<Item = &'a <R as Region>::Index> + Clone,
+{
     fn clone(&self) -> Self {
         Self {
             inner: self.inner.clone(),