diff --git a/src/snapshot_vec.rs b/src/snapshot_vec.rs
index 1c3643d..7bb68a2 100644
--- a/src/snapshot_vec.rs
+++ b/src/snapshot_vec.rs
@@ -60,8 +60,10 @@ impl<D> fmt::Debug for SnapshotVec<D>
 
 // Snapshots are tokens that should be created/consumed linearly.
 pub struct Snapshot {
+    // Number of values at the time the snapshot was taken.
+    pub(crate) value_count: usize,
     // Length of the undo log at the time the snapshot was taken.
-    pub(crate) length: usize,
+    undo_len: usize,
 }
 
 pub trait SnapshotVecDelegate {
@@ -173,27 +175,29 @@ impl<D: SnapshotVecDelegate> SnapshotVec<D> {
     }
 
     pub fn start_snapshot(&mut self) -> Snapshot {
-        let length = self.undo_log.len();
         self.num_open_snapshots += 1;
-        Snapshot { length: length }
+        Snapshot {
+            value_count: self.values.len(),
+            undo_len: self.undo_log.len(),
+        }
     }
 
     pub fn actions_since_snapshot(&self, snapshot: &Snapshot) -> &[UndoLog<D>] {
-        &self.undo_log[snapshot.length..]
+        &self.undo_log[snapshot.undo_len..]
     }
 
     fn assert_open_snapshot(&self, snapshot: &Snapshot) {
         // Failures here may indicate a failure to follow a stack discipline.
-        assert!(self.undo_log.len() >= snapshot.length);
+        assert!(self.undo_log.len() >= snapshot.undo_len);
         assert!(self.num_open_snapshots > 0);
     }
 
     pub fn rollback_to(&mut self, snapshot: Snapshot) {
-        debug!("rollback_to({})", snapshot.length);
+        debug!("rollback_to({})", snapshot.undo_len);
 
         self.assert_open_snapshot(&snapshot);
 
-        while self.undo_log.len() > snapshot.length {
+        while self.undo_log.len() > snapshot.undo_len {
             match self.undo_log.pop().unwrap() {
                 NewElem(i) => {
                     self.values.pop();
@@ -216,7 +220,7 @@ impl<D: SnapshotVecDelegate> SnapshotVec<D> {
     /// Commits all changes since the last snapshot. Of course, they
     /// can still be undone if there is a snapshot further out.
     pub fn commit(&mut self, snapshot: Snapshot) {
-        debug!("commit({})", snapshot.length);
+        debug!("commit({})", snapshot.undo_len);
 
         self.assert_open_snapshot(&snapshot);
 
@@ -224,7 +228,7 @@ impl<D: SnapshotVecDelegate> SnapshotVec<D> {
             // The root snapshot. It's safe to clear the undo log because
             // there's no snapshot further out that we might need to roll back
             // to.
-            assert!(snapshot.length == 0);
+            assert!(snapshot.undo_len == 0);
             self.undo_log.clear();
         }
 
diff --git a/src/unify/backing_vec.rs b/src/unify/backing_vec.rs
index e3e91ca..ea4cca5 100644
--- a/src/unify/backing_vec.rs
+++ b/src/unify/backing_vec.rs
@@ -1,8 +1,7 @@
 #[cfg(feature = "persistent")]
 use dogged::DVec;
 use snapshot_vec as sv;
-use std::ops;
-use std::ops::RangeInclusive;
+use std::ops::{self, Range};
 use std::marker::PhantomData;
 
 use super::{VarValue, UnifyKey, UnifyValue};
@@ -11,19 +10,15 @@ use super::{VarValue, UnifyKey, UnifyValue};
 #[allow(type_alias_bounds)]
 type Key<S: UnificationStore> = <S as UnificationStore>::Key;
 
-pub trait Measurable {
-    fn len(&self) -> usize;
-}
-
 /// Largely internal trait implemented by the unification table
 /// backing store types. The most common such type is `InPlace`,
 /// which indicates a standard, mutable unification table.
 pub trait UnificationStore:
-    ops::Index<usize, Output = VarValue<Key<Self>>> + Measurable + Clone + Default
+    ops::Index<usize, Output = VarValue<Key<Self>>> + Clone + Default
 {
     type Key: UnifyKey<Value = Self::Value>;
     type Value: UnifyValue;
-    type Snapshot: Measurable;
+    type Snapshot;
 
     fn start_snapshot(&mut self) -> Self::Snapshot;
 
@@ -31,15 +26,15 @@ pub trait UnificationStore:
 
     fn commit(&mut self, snapshot: Self::Snapshot);
 
-    fn values_since_snapshot(&mut self, snapshot: &Self::Snapshot) -> RangeInclusive<usize> {
-        snapshot.len()..=self.len()
-    }
+    fn values_since_snapshot(&self, snapshot: &Self::Snapshot) -> Range<usize>;
 
     fn reset_unifications(
         &mut self,
         value: impl FnMut(u32) -> VarValue<Self::Key>,
     );
 
+    fn len(&self) -> usize;
+
     fn push(&mut self, value: VarValue<Self::Key>);
 
     fn reserve(&mut self, num_new_values: usize);
@@ -66,20 +61,6 @@ impl<K: UnifyKey> Default for InPlace<K> {
     }
 }
 
-impl Measurable for sv::Snapshot {
-    #[inline]
-    fn len(&self) -> usize {
-        self.length
-    }
-}
-
-impl<K: UnifyKey> Measurable for InPlace<K> {
-    #[inline]
-    fn len(&self) -> usize {
-        self.values.len()
-    }
-}
-
 impl<K: UnifyKey> UnificationStore for InPlace<K> {
     type Key = K;
     type Value = K::Value;
@@ -100,6 +81,11 @@ impl<K: UnifyKey> UnificationStore for InPlace<K> {
         self.values.commit(snapshot);
     }
 
+    #[inline]
+    fn values_since_snapshot(&self, snapshot: &Self::Snapshot) -> Range<usize> {
+        snapshot.value_count..self.len()
+    }
+
     #[inline]
     fn reset_unifications(
         &mut self,
@@ -108,6 +94,10 @@ impl<K: UnifyKey> UnificationStore for InPlace<K> {
         self.values.set_all(|i| value(i as u32));
     }
 
+    fn len(&self) -> usize {
+        self.values.len()
+    }
+
     #[inline]
     fn push(&mut self, value: VarValue<Self::Key>) {
         self.values.push(value);
@@ -159,14 +149,6 @@ impl<K: UnifyKey> Default for Persistent<K> {
     }
 }
 
-#[cfg(feature = "persistent")]
-impl<K: UnifyKey> Measurable for Persistent<K> {
-    #[inline]
-    fn len(&self) -> usize {
-        self.values.len()
-    }
-}
-
 #[cfg(feature = "persistent")]
 impl<K: UnifyKey> UnificationStore for Persistent<K> {
     type Key = K;
@@ -184,7 +166,11 @@ impl<K: UnifyKey> UnificationStore for Persistent<K> {
     }
 
     #[inline]
-    fn commit(&mut self, _snapshot: Self::Snapshot) {
+    fn commit(&mut self, _snapshot: Self::Snapshot) {}
+
+    #[inline]
+    fn values_since_snapshot(&self, snapshot: &Self::Snapshot) -> Range<usize> {
+        snapshot.len()..self.len()
     }
 
     #[inline]
@@ -200,6 +186,10 @@ impl<K: UnifyKey> UnificationStore for Persistent<K> {
         }
     }
 
+    fn len(&self) -> usize {
+        self.values.len()
+    }
+
     #[inline]
     fn push(&mut self, value: VarValue<Self::Key>) {
         self.values.push(value);
diff --git a/src/unify/mod.rs b/src/unify/mod.rs
index d8e871a..dbef13a 100644
--- a/src/unify/mod.rs
+++ b/src/unify/mod.rs
@@ -33,6 +33,7 @@
 
 use std::marker;
 use std::fmt::Debug;
+use std::ops::Range;
 
 mod backing_vec;
 pub use self::backing_vec::{InPlace, UnificationStore};
@@ -299,6 +300,15 @@ impl<S: UnificationStore> UnificationTable<S> {
         self.values.len()
     }
 
+    /// Returns the keys of all variables created since the `snapshot`.
+    pub fn vars_since_snapshot(
+        &self,
+        snapshot: &Snapshot<S>,
+    ) -> Range<S::Key> {
+        let range = self.values.values_since_snapshot(&snapshot.snapshot);
+        S::Key::from_index(range.start as u32)..S::Key::from_index(range.end as u32)
+    }
+
     /// Obtains the current value for a particular key.
     /// Not for end-users; they can use `probe_value`.
     fn value(&self, key: S::Key) -> &VarValue<S::Key> {