diff --git a/src/crates/benchmarker/Cargo.toml b/src/crates/benchmarker/Cargo.toml
index 3fffe5e..875ed26 100644
--- a/src/crates/benchmarker/Cargo.toml
+++ b/src/crates/benchmarker/Cargo.toml
@@ -17,3 +17,7 @@ harness = false
 [[bench]]
 name = "linked_list"
 harness = false
+
+[[bench]]
+name = "hashmap"
+harness = false
diff --git a/src/crates/benchmarker/benches/hashmap.rs b/src/crates/benchmarker/benches/hashmap.rs
new file mode 100644
index 0000000..dbd498a
--- /dev/null
+++ b/src/crates/benchmarker/benches/hashmap.rs
@@ -0,0 +1,7 @@
+fn main() {
+    let ns = [
+        64, 128, 256, 512, 1024, 2048, 3072, 4096, 5120, 6144, 7168, 8192, 16384, 24576, 32768,
+        40960, 49152, 57344, 65536,
+    ];
+    candelabra_benchmarker::benchmark_mapping::<std::collections::HashMap<usize, usize>, _, _>(&ns);
+}
diff --git a/src/crates/benchmarker/src/lib.rs b/src/crates/benchmarker/src/lib.rs
index 3018f56..f449527 100644
--- a/src/crates/benchmarker/src/lib.rs
+++ b/src/crates/benchmarker/src/lib.rs
@@ -1,5 +1,6 @@
 mod container;
 mod indexable;
+mod mapping;
 mod stack;
 
 use std::{
@@ -9,6 +10,7 @@ use std::{
 
 pub use container::*;
 pub use indexable::*;
+pub use mapping::*;
 pub use stack::*;
 
 const WARM_UP_TIME: Duration = Duration::from_millis(500);
diff --git a/src/crates/benchmarker/src/mapping.rs b/src/crates/benchmarker/src/mapping.rs
new file mode 100644
index 0000000..b567b30
--- /dev/null
+++ b/src/crates/benchmarker/src/mapping.rs
@@ -0,0 +1,141 @@
+use primrose_library::traits::Mapping;
+use rand::{
+    distributions::{Distribution, Standard},
+    random, thread_rng, Rng,
+};
+
+use crate::{benchmark_op, print_result};
+
+pub fn benchmark_mapping<T, K, V>(ns: &[usize])
+where
+    T: Mapping<K, V> + Default + Clone,
+    K: Copy,
+    V: Copy,
+    Standard: Distribution<K> + Distribution<V>,
+{
+    for n in ns {
+        scenario_populate::<T, K, V>(*n);
+        scenario_contains::<T, K, V>(*n);
+        scenario_remove::<T, K, V>(*n);
+        scenario_clear::<T, K, V>(*n);
+    }
+}
+
+fn scenario_populate<T, K, V>(n: usize)
+where
+    T: Mapping<K, V> + Default + Clone,
+    K: Copy,
+    V: Copy,
+    Standard: Distribution<K> + Distribution<V>,
+{
+    let mut results = benchmark_op(
+        || {
+            (
+                T::default(),
+                (0..n)
+                    .map(|_| (random(), random()))
+                    .collect::<Vec<(K, V)>>(),
+            )
+        },
+        |(c, xs)| {
+            for (k, v) in xs {
+                c.insert(*k, *v);
+            }
+        },
+    );
+
+    // Since we've repeated n times in each run
+    results.min_nanos /= n as f64;
+    results.avg_nanos /= n as f64;
+    results.max_nanos /= n as f64;
+
+    print_result("insert", n, results);
+}
+
+fn scenario_contains<T, K, V>(n: usize)
+where
+    T: Mapping<K, V> + Default + Clone,
+    K: Copy,
+    V: Copy,
+    Standard: Distribution<K> + Distribution<V>,
+{
+    let results = benchmark_op(
+        || {
+            let mut rng = thread_rng();
+            let mut c = T::default();
+
+            // decide where the element that we will search for will be
+            let pivot = rng.gen_range(0..n);
+
+            // insert the element at pivot, and keep track of what it is
+            for _ in 0..pivot {
+                c.insert(random(), random());
+            }
+            let chosen = rng.gen();
+            c.insert(chosen, random());
+            for _ in pivot..n {
+                c.insert(random(), random());
+            }
+
+            (c, chosen)
+        },
+        |(c, chosen)| c.contains(chosen),
+    );
+
+    print_result("contains", n, results);
+}
+
+fn scenario_remove<T, K, V>(n: usize)
+where
+    T: Mapping<K, V> + Default + Clone,
+    K: Copy,
+    V: Copy,
+    Standard: Distribution<K> + Distribution<V>,
+{
+    let results = benchmark_op(
+        || {
+            let mut rng = thread_rng();
+            let mut c = T::default();
+
+            // decide where the element that we will search for will be
+            let pivot = rng.gen_range(0..n);
+
+            // insert the element at pivot, and keep track of what it is
+            for _ in 0..pivot {
+                c.insert(random(), random());
+            }
+            let chosen = rng.gen();
+            c.insert(chosen, random());
+            for _ in pivot..n {
+                c.insert(random(), random());
+            }
+
+            (c, chosen)
+        },
+        |(c, chosen)| c.remove(chosen),
+    );
+
+    print_result("remove", n, results);
+}
+
+fn scenario_clear<T, K, V>(n: usize)
+where
+    T: Mapping<K, V> + Default + Clone,
+    K: Copy,
+    V: Copy,
+    Standard: Distribution<K> + Distribution<V>,
+{
+    let results = benchmark_op(
+        || {
+            let mut c = T::default();
+
+            for _ in 0..n {
+                c.insert(random(), random());
+            }
+            c
+        },
+        |c| c.clear(),
+    );
+
+    print_result("clear", n, results);
+}
diff --git a/src/crates/library/src/eager_sorted_vector.rs b/src/crates/library/src/eager_sorted_vector.rs
index 9be45d0..bf67105 100644
--- a/src/crates/library/src/eager_sorted_vector.rs
+++ b/src/crates/library/src/eager_sorted_vector.rs
@@ -23,7 +23,7 @@ impl<T: Ord> EagerSortedVec<T> {
         EagerSortedVec { v: Vec::new() }
     }
 
-    pub fn len(&mut self) -> usize {
+    pub fn len(&self) -> usize {
         self.v.len()
     }
 
@@ -31,7 +31,7 @@ impl<T: Ord> EagerSortedVec<T> {
         self.v.binary_search(x).is_ok()
     }
 
-    pub fn is_empty(&mut self) -> bool {
+    pub fn is_empty(&self) -> bool {
         self.len() == 0
     }
 
@@ -81,7 +81,7 @@ impl<T: Ord> Container<T> for EagerSortedVec<T> {
     (define (pre-len xs) (equal? xs (sort xs <)))
     (define (post-len xs r) (equal? r (op-len xs)))
     *ENDLIBSPEC*/
-    fn len(&mut self) -> usize {
+    fn len(&self) -> usize {
         EagerSortedVec::len(self)
     }
 
@@ -108,7 +108,7 @@ impl<T: Ord> Container<T> for EagerSortedVec<T> {
     (define (pre-is-empty xs) (equal? xs (sort xs <)))
     (define (post-is-empty xs r) (equal? r (op-is-empty xs)))
     *ENDLIBSPEC*/
-    fn is_empty(&mut self) -> bool {
+    fn is_empty(&self) -> bool {
         EagerSortedVec::is_empty(self)
     }
 
diff --git a/src/crates/library/src/eager_unique_vector.rs b/src/crates/library/src/eager_unique_vector.rs
index b5e0284..632762b 100644
--- a/src/crates/library/src/eager_unique_vector.rs
+++ b/src/crates/library/src/eager_unique_vector.rs
@@ -28,7 +28,7 @@ impl<T: PartialEq> EagerUniqueVec<T> {
         EagerUniqueVec { v: Vec::new() }
     }
 
-    pub fn len(&mut self) -> usize {
+    pub fn len(&self) -> usize {
         self.v.len()
     }
 
@@ -36,7 +36,7 @@ impl<T: PartialEq> EagerUniqueVec<T> {
         self.v.contains(x)
     }
 
-    pub fn is_empty(&mut self) -> bool {
+    pub fn is_empty(&self) -> bool {
         self.len() == 0
     }
 
@@ -88,7 +88,7 @@ impl<T: Ord + PartialEq> Container<T> for EagerUniqueVec<T> {
     (define (pre-len xs) (equal? xs (remove-duplicates xs)))
     (define (post-len xs r) (equal? r (op-len xs)))
     *ENDLIBSPEC*/
-    fn len(&mut self) -> usize {
+    fn len(&self) -> usize {
         EagerUniqueVec::len(self)
     }
 
@@ -115,7 +115,7 @@ impl<T: Ord + PartialEq> Container<T> for EagerUniqueVec<T> {
     (define (pre-is-empty xs) (equal? xs (remove-duplicates xs)))
     (define (post-is-empty xs r) (equal? r (op-is-empty xs)))
     *ENDLIBSPEC*/
-    fn is_empty(&mut self) -> bool {
+    fn is_empty(&self) -> bool {
         EagerUniqueVec::is_empty(self)
     }
 
diff --git a/src/crates/library/src/hashmap.rs b/src/crates/library/src/hashmap.rs
new file mode 100644
index 0000000..ace6453
--- /dev/null
+++ b/src/crates/library/src/hashmap.rs
@@ -0,0 +1,41 @@
+/*LIBSPEC-NAME*
+rust-hashmap-spec std::collections::HashMap
+*ENDLIBSPEC-NAME*/
+
+use crate::traits::Mapping;
+pub use std::collections::HashMap;
+use std::hash::Hash;
+
+/*IMPL*
+Mapping
+*ENDIMPL*/
+impl<K: Ord + Hash, V> Mapping<K, V> for HashMap<K, V> {
+    fn len(&self) -> usize {
+        HashMap::len(self)
+    }
+
+    fn contains(&mut self, x: &K) -> bool {
+        HashMap::contains_key(self, x)
+    }
+
+    fn insert(&mut self, key: K, val: V) -> Option<V> {
+        HashMap::insert(self, key, val)
+    }
+
+    fn get(&mut self, key: &K) -> Option<&V> {
+        HashMap::get(self, key)
+    }
+
+    fn remove(&mut self, key: &K) -> Option<V> {
+        HashMap::remove(self, key)
+    }
+
+    fn clear(&mut self) {
+        HashMap::clear(self)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    // TODO
+}
diff --git a/src/crates/library/src/hashset.rs b/src/crates/library/src/hashset.rs
index d7d8915..f8f637c 100644
--- a/src/crates/library/src/hashset.rs
+++ b/src/crates/library/src/hashset.rs
@@ -18,7 +18,7 @@ impl<T: Ord + Hash> Container<T> for HashSet<T> {
     (define (pre-len xs) (equal? xs (remove-duplicates (sort xs <))))
     (define (post-len xs r) (equal? r (op-len xs)))
     *ENDLIBSPEC*/
-    fn len(&mut self) -> usize {
+    fn len(&self) -> usize {
         HashSet::len(self)
     }
 
@@ -45,7 +45,7 @@ impl<T: Ord + Hash> Container<T> for HashSet<T> {
     (define (pre-is-empty xs) (equal? xs (remove-duplicates (sort xs <))))
     (define (post-is-empty xs r) (equal? r (op-is-empty xs)))
     *ENDLIBSPEC*/
-    fn is_empty(&mut self) -> bool {
+    fn is_empty(&self) -> bool {
         HashSet::is_empty(self)
     }
 
diff --git a/src/crates/library/src/lazy_sorted_vector.rs b/src/crates/library/src/lazy_sorted_vector.rs
index 5ead065..17cb7f6 100644
--- a/src/crates/library/src/lazy_sorted_vector.rs
+++ b/src/crates/library/src/lazy_sorted_vector.rs
@@ -27,11 +27,7 @@ impl<T: Ord> LazySortedVec<T> {
         }
     }
 
-    pub fn len(&mut self) -> usize {
-        if self.modified {
-            self.v.sort();
-            self.modified = false;
-        }
+    pub fn len(&self) -> usize {
         self.v.len()
     }
 
@@ -43,11 +39,7 @@ impl<T: Ord> LazySortedVec<T> {
         self.v.binary_search(x).is_ok()
     }
 
-    pub fn is_empty(&mut self) -> bool {
-        if self.modified {
-            self.v.sort();
-            self.modified = false;
-        }
+    pub fn is_empty(&self) -> bool {
         self.len() == 0
     }
 
@@ -117,7 +109,7 @@ impl<T: Ord> Container<T> for LazySortedVec<T> {
     (define (pre-len xs) (equal? xs (sort xs <)))
     (define (post-len xs r) (equal? r (op-len xs)))
     *ENDLIBSPEC*/
-    fn len(&mut self) -> usize {
+    fn len(&self) -> usize {
         LazySortedVec::len(self)
     }
 
@@ -144,7 +136,7 @@ impl<T: Ord> Container<T> for LazySortedVec<T> {
     (define (pre-is-empty xs) (equal? xs (sort xs <)))
     (define (post-is-empty xs r) (equal? r (op-is-empty xs)))
     *ENDLIBSPEC*/
-    fn is_empty(&mut self) -> bool {
+    fn is_empty(&self) -> bool {
         LazySortedVec::is_empty(self)
     }
 
diff --git a/src/crates/library/src/lazy_unique_vector.rs b/src/crates/library/src/lazy_unique_vector.rs
index 9b5e60c..e9c24dc 100644
--- a/src/crates/library/src/lazy_unique_vector.rs
+++ b/src/crates/library/src/lazy_unique_vector.rs
@@ -28,12 +28,7 @@ impl<T: Ord> LazyUniqueVec<T> {
         LazyUniqueVec { v, modified: false }
     }
 
-    pub fn len(&mut self) -> usize {
-        if self.modified {
-            self.v.sort();
-            self.v.dedup();
-            self.modified = false;
-        }
+    pub fn len(&self) -> usize {
         self.v.len()
     }
 
@@ -46,12 +41,7 @@ impl<T: Ord> LazyUniqueVec<T> {
         self.v.binary_search(x).is_ok()
     }
 
-    pub fn is_empty(&mut self) -> bool {
-        if self.modified {
-            self.v.sort();
-            self.v.dedup();
-            self.modified = false;
-        }
+    pub fn is_empty(&self) -> bool {
         self.len() == 0
     }
 
@@ -127,7 +117,7 @@ impl<T: Ord> Container<T> for LazyUniqueVec<T> {
     (define (pre-len xs) (equal? xs (remove-duplicates (sort xs <))))
     (define (post-len xs r) (equal? r (op-len xs)))
     *ENDLIBSPEC*/
-    fn len(&mut self) -> usize {
+    fn len(&self) -> usize {
         LazyUniqueVec::len(self)
     }
 
@@ -154,7 +144,7 @@ impl<T: Ord> Container<T> for LazyUniqueVec<T> {
     (define (pre-is-empty xs) (equal? xs (remove-duplicates (sort xs <))))
     (define (post-is-empty xs r) (equal? r (op-is-empty xs)))
     *ENDLIBSPEC*/
-    fn is_empty(&mut self) -> bool {
+    fn is_empty(&self) -> bool {
         LazyUniqueVec::is_empty(self)
     }
 
diff --git a/src/crates/library/src/lib.rs b/src/crates/library/src/lib.rs
index 1c39482..db5aac7 100644
--- a/src/crates/library/src/lib.rs
+++ b/src/crates/library/src/lib.rs
@@ -12,6 +12,7 @@ mod eager_unique_vector;
 mod lazy_sorted_vector;
 mod lazy_unique_vector;
 
+mod hashmap;
 mod hashset;
 mod list;
 mod treeset;
diff --git a/src/crates/library/src/list.rs b/src/crates/library/src/list.rs
index 191b112..4ee045c 100644
--- a/src/crates/library/src/list.rs
+++ b/src/crates/library/src/list.rs
@@ -19,7 +19,7 @@ impl<T: Ord> Container<T> for LinkedList<T> {
     (define (pre-len xs) #t)
     (define (post-len xs r) (equal? r (op-len xs)))
     *ENDLIBSPEC*/
-    fn len(&mut self) -> usize {
+    fn len(&self) -> usize {
         LinkedList::len(self)
     }
 
@@ -46,7 +46,7 @@ impl<T: Ord> Container<T> for LinkedList<T> {
     (define (pre-is-empty xs) #t)
     (define (post-is-empty xs r) (equal? r (op-is-empty xs)))
     *ENDLIBSPEC*/
-    fn is_empty(&mut self) -> bool {
+    fn is_empty(&self) -> bool {
         LinkedList::is_empty(self)
     }
 
diff --git a/src/crates/library/src/profiler.rs b/src/crates/library/src/profiler.rs
index 952d0a0..ba8c357 100644
--- a/src/crates/library/src/profiler.rs
+++ b/src/crates/library/src/profiler.rs
@@ -2,12 +2,13 @@ use std::{
     env::var,
     fs::{create_dir, metadata, File},
     io::Write,
+    marker::PhantomData,
     time::SystemTime,
 };
 
-use crate::traits::{Container, Indexable, Stack};
+use crate::traits::{Container, Indexable, Mapping, Stack};
 
-pub struct ProfilerWrapper<const ID: usize, T> {
+pub struct ProfilerWrapper<const ID: usize, T, E> {
     inner: T,
     sum_ns: usize,
     n_contains: usize,
@@ -19,9 +20,11 @@ pub struct ProfilerWrapper<const ID: usize, T> {
     n_nth: usize,
     n_push: usize,
     n_pop: usize,
+    n_get: usize,
+    _d: PhantomData<E>,
 }
 
-impl<const ID: usize, T: Default> Default for ProfilerWrapper<ID, T> {
+impl<const ID: usize, T: Default, E> Default for ProfilerWrapper<ID, T, E> {
     fn default() -> Self {
         Self {
             inner: T::default(),
@@ -35,79 +38,131 @@ impl<const ID: usize, T: Default> Default for ProfilerWrapper<ID, T> {
             n_nth: 0,
             n_push: 0,
             n_pop: 0,
+            n_get: 0,
+            _d: Default::default(),
         }
     }
 }
 
-impl<const ID: usize, T: Container<E>, E> Container<E> for ProfilerWrapper<ID, T> {
-    fn len(&mut self) -> usize {
+impl<const ID: usize, T: Container<E>, E> ProfilerWrapper<ID, T, E> {
+    fn add_n(&mut self) {
+        self.sum_ns += self.inner.len();
+    }
+}
+
+impl<const ID: usize, T: Mapping<K, V>, K, V> ProfilerWrapper<ID, T, (K, V)> {
+    fn add_n_map(&mut self) {
+        self.sum_ns += self.inner.len();
+    }
+}
+
+impl<const ID: usize, T: Container<E>, E> Container<E> for ProfilerWrapper<ID, T, E> {
+    fn len(&self) -> usize {
         self.inner.len()
     }
 
     fn contains(&mut self, x: &E) -> bool {
-        self.sum_ns += self.inner.len();
+        self.add_n();
         self.n_contains += 1;
         self.inner.contains(x)
     }
 
-    fn is_empty(&mut self) -> bool {
+    fn is_empty(&self) -> bool {
         self.inner.is_empty()
     }
 
     fn insert(&mut self, elt: E) {
-        self.sum_ns += self.inner.len();
+        self.add_n();
         self.n_insert += 1;
         self.inner.insert(elt)
     }
 
     fn clear(&mut self) {
-        self.sum_ns += self.inner.len();
+        self.add_n();
         self.n_clear += 1;
         self.inner.clear()
     }
 
     fn remove(&mut self, elt: E) -> Option<E> {
-        self.sum_ns += self.inner.len();
+        self.add_n();
         self.n_remove += 1;
         self.inner.remove(elt)
     }
 }
 
-impl<const ID: usize, T: Indexable<E> + Container<E>, E> Indexable<E> for ProfilerWrapper<ID, T> {
+impl<const ID: usize, T: Indexable<E> + Container<E>, E> Indexable<E>
+    for ProfilerWrapper<ID, T, E>
+{
     fn first(&mut self) -> Option<&E> {
-        self.sum_ns += self.inner.len();
+        self.add_n();
         self.n_first += 1;
         self.inner.first()
     }
 
     fn last(&mut self) -> Option<&E> {
-        self.sum_ns += self.inner.len();
+        self.add_n();
         self.n_last += 1;
         self.inner.last()
     }
 
     fn nth(&mut self, n: usize) -> Option<&E> {
-        self.sum_ns += self.inner.len();
+        self.add_n();
         self.n_nth += 1;
         self.inner.nth(n)
     }
 }
 
-impl<const ID: usize, T: Stack<E> + Container<E>, E> Stack<E> for ProfilerWrapper<ID, T> {
+impl<const ID: usize, T: Stack<E> + Container<E>, E> Stack<E> for ProfilerWrapper<ID, T, E> {
     fn push(&mut self, elt: E) {
-        self.sum_ns += self.inner.len();
+        self.add_n();
         self.n_push += 1;
         self.inner.push(elt)
     }
 
     fn pop(&mut self) -> Option<E> {
-        self.sum_ns += self.inner.len();
+        self.add_n();
         self.n_pop += 1;
         self.inner.pop()
     }
 }
 
-impl<const ID: usize, T> Drop for ProfilerWrapper<ID, T> {
+impl<const ID: usize, T: Mapping<K, V>, K, V> Mapping<K, V> for ProfilerWrapper<ID, T, (K, V)> {
+    fn len(&self) -> usize {
+        self.inner.len()
+    }
+
+    fn contains(&mut self, x: &K) -> bool {
+        self.add_n_map();
+        self.n_contains += 1;
+        self.inner.contains(x)
+    }
+
+    fn insert(&mut self, key: K, val: V) -> Option<V> {
+        self.add_n_map();
+        self.n_insert += 1;
+        self.inner.insert(key, val)
+    }
+
+    fn get(&mut self, key: &K) -> Option<&V> {
+        self.add_n_map();
+        self.n_get += 1;
+        self.inner.get(key)
+    }
+
+    fn remove(&mut self, key: &K) -> Option<V> {
+        self.add_n_map();
+        self.n_remove += 1;
+        self.inner.remove(key)
+    }
+
+    fn clear(&mut self) {
+        self.add_n_map();
+        self.n_clear += 1;
+        self.inner.clear()
+    }
+}
+
+impl<const ID: usize, T, E> Drop for ProfilerWrapper<ID, T, E> {
     fn drop(&mut self) {
         let base_dir =
             var("PROFILER_OUT_DIR").expect("ProfilerWrapper used without environment variable");
@@ -130,5 +185,6 @@ impl<const ID: usize, T> Drop for ProfilerWrapper<ID, T> {
         writeln!(f, "{}", self.n_nth).unwrap();
         writeln!(f, "{}", self.n_push).unwrap();
         writeln!(f, "{}", self.n_pop).unwrap();
+        writeln!(f, "{}", self.n_get).unwrap();
     }
 }
diff --git a/src/crates/library/src/traits.rs b/src/crates/library/src/traits.rs
index a29f7e0..bf4616a 100644
--- a/src/crates/library/src/traits.rs
+++ b/src/crates/library/src/traits.rs
@@ -1,10 +1,10 @@
 //! Common traits for primrose container types
 pub trait Container<T>: Default {
     /// Get the current number of elements
-    fn len(&mut self) -> usize;
+    fn len(&self) -> usize;
 
     /// Check if the container is empty
-    fn is_empty(&mut self) -> bool;
+    fn is_empty(&self) -> bool;
 
     /// Check if the specified item is in the container
     fn contains(&mut self, x: &T) -> bool;
@@ -29,3 +29,28 @@ pub trait Indexable<T> {
     fn last(&mut self) -> Option<&T>;
     fn nth(&mut self, n: usize) -> Option<&T>;
 }
+
+pub trait Mapping<K, V>: Default {
+    /// Get the current number of elements
+    fn len(&self) -> usize;
+
+    /// Check if the container is empty
+    fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// Check if the specified item is in the container
+    fn contains(&mut self, x: &K) -> bool;
+
+    /// Insert the given item
+    fn insert(&mut self, key: K, val: V) -> Option<V>;
+
+    /// Get the value for the given key
+    fn get(&mut self, key: &K) -> Option<&V>;
+
+    /// Remove the first occurence of the given item
+    fn remove(&mut self, key: &K) -> Option<V>;
+
+    /// Remove all elements from the container
+    fn clear(&mut self);
+}
diff --git a/src/crates/library/src/treeset.rs b/src/crates/library/src/treeset.rs
index 4e2b213..f4f6923 100644
--- a/src/crates/library/src/treeset.rs
+++ b/src/crates/library/src/treeset.rs
@@ -17,7 +17,7 @@ impl<T: Ord> Container<T> for BTreeSet<T> {
     (define (pre-len xs) (equal? xs (remove-duplicates (sort xs <))))
     (define (post-len xs r) (equal? r (op-len xs)))
     *ENDLIBSPEC*/
-    fn len(&mut self) -> usize {
+    fn len(&self) -> usize {
         BTreeSet::len(self)
     }
 
@@ -44,7 +44,7 @@ impl<T: Ord> Container<T> for BTreeSet<T> {
     (define (pre-is-empty xs) (equal? xs (remove-duplicates (sort xs <))))
     (define (post-is-empty xs r) (equal? r (op-is-empty xs)))
     *ENDLIBSPEC*/
-    fn is_empty(&mut self) -> bool {
+    fn is_empty(&self) -> bool {
         BTreeSet::is_empty(self)
     }
 
diff --git a/src/crates/library/src/vector.rs b/src/crates/library/src/vector.rs
index 38293c8..496f4f0 100644
--- a/src/crates/library/src/vector.rs
+++ b/src/crates/library/src/vector.rs
@@ -17,7 +17,7 @@ impl<T: PartialEq> Container<T> for Vec<T> {
     (define (pre-len xs) #t)
     (define (post-len xs r) (equal? r (op-len xs)))
     *ENDLIBSPEC*/
-    fn len(&mut self) -> usize {
+    fn len(&self) -> usize {
         Vec::len(self)
     }
 
@@ -44,7 +44,7 @@ impl<T: PartialEq> Container<T> for Vec<T> {
     (define (pre-is-empty xs) #t)
     (define (post-is-empty xs r) (equal? r (op-is-empty xs)))
     *ENDLIBSPEC*/
-    fn is_empty(&mut self) -> bool {
+    fn is_empty(&self) -> bool {
         Vec::is_empty(self)
     }