diff --git a/Cargo.toml b/Cargo.toml
index 4b0c12f..fa9a029 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "orx-linked-list"
-version = "4.0.0"
+version = "4.1.0"
 edition = "2024"
 authors = ["orxfun <orx.ugur.arikan@gmail.com>"]
 description = "A linked list implementation with unique features and an extended list of constant time methods providing high performance traversals and mutations."
@@ -16,7 +16,7 @@ orx-pinned-vec = { version = "3.21.0", default-features = false }
 orx-fixed-vec = { version = "3.22.0", default-features = false }
 orx-split-vec = { version = "3.22.0", default-features = false }
 orx-concurrent-iter = { version = "3.3.0", default-features = false }
-orx-selfref-col = { version = "3.0.0", default-features = false }
+orx-selfref-col = { version = "3.1.0", default-features = false }
 orx-parallel = { version = "3.4.0", default-features = false, optional = true }
 
 [dev-dependencies]
diff --git a/examples/bench_parallelization.rs b/examples/bench_parallelization.rs
index fbfcdba..931a927 100644
--- a/examples/bench_parallelization.rs
+++ b/examples/bench_parallelization.rs
@@ -33,25 +33,28 @@ fn main() {
     let args = Args::parse();
 
     let expected_output = {
-        let list: DoublyList<_> = (0..args.len as usize).collect();
+        let list: DoublyList<_> = (0..args.len).collect();
 
         list.iter()
             .filter(|x| *x % 3 != 0)
             .map(|x| x + fibonacci(x % 1000))
-            .filter_map(|x| (x % 2 == 0).then(|| x.to_string()))
+            .filter(|&x| x % 2 == 0)
+            .map(|x| x.to_string())
             .collect::<Vec<_>>()
     };
 
-    let computations: Vec<(&str, Box<dyn Fn() -> Vec<String>>)> = vec![
+    type Compute = dyn Fn() -> Vec<String>;
+    let computations: Vec<(&str, Box<Compute>)> = vec![
         (
             "Sequential computation over std::collections::LinkedList",
             Box::new(move || {
-                let list: std::collections::LinkedList<_> = (0..args.len as usize).collect();
+                let list: std::collections::LinkedList<_> = (0..args.len).collect();
 
                 list.iter()
                     .filter(|x| *x % 3 != 0)
                     .map(|x| x + fibonacci(x % 1000))
-                    .filter_map(|x| (x % 2 == 0).then(|| x.to_string()))
+                    .filter(|&x| x % 2 == 0)
+                    .map(|x| x.to_string())
                     .collect::<Vec<_>>()
             }),
         ),
@@ -59,12 +62,13 @@ fn main() {
         (
             "Sequential computation over DoublyList",
             Box::new(move || {
-                let list: DoublyList<_> = (0..args.len as usize).collect();
+                let list: DoublyList<_> = (0..args.len).collect();
 
                 list.iter_x()
                     .filter(|x| *x % 3 != 0)
                     .map(|x| x + fibonacci(x % 1000))
-                    .filter_map(|x| (x % 2 == 0).then(|| x.to_string()))
+                    .filter(|&x| x % 2 == 0)
+                    .map(|x| x.to_string())
                     .collect::<Vec<_>>()
             }),
         ),
@@ -72,7 +76,7 @@ fn main() {
         (
             "Parallelized over DoublyList using orx_parallel",
             Box::new(move || {
-                let list: DoublyList<_> = (0..args.len as usize).collect();
+                let list: DoublyList<_> = (0..args.len).collect();
 
                 list.par_x() // replace iter_x (into_iter_x) with par_x (into_par_x) to parallelize !
                     .filter(|x| *x % 3 != 0)
diff --git a/examples/tour_mutations.rs b/examples/tour_mutations.rs
index 1161cd2..5c4b417 100644
--- a/examples/tour_mutations.rs
+++ b/examples/tour_mutations.rs
@@ -2,7 +2,7 @@ use clap::Parser;
 use orx_linked_list::*;
 use rand::prelude::*;
 use rand_chacha::ChaCha8Rng;
-use std::{fmt::Debug, time::Instant, usize};
+use std::{fmt::Debug, time::Instant};
 
 fn get_cities(num_cities: usize) -> impl Iterator<Item = City> {
     (0..num_cities).map(|id| City {
diff --git a/examples/utils/benchmark_utils.rs b/examples/utils/benchmark_utils.rs
index 4ab3241..4f7f2ab 100644
--- a/examples/utils/benchmark_utils.rs
+++ b/examples/utils/benchmark_utils.rs
@@ -35,11 +35,13 @@ where
     now.elapsed().unwrap()
 }
 
+type Compute<O> = dyn Fn() -> O;
+
 pub fn timed_reduce_all<O>(
     benchmark_name: &str,
     num_repetitions: usize,
     expected_output: Option<O>,
-    computations: &[(&str, Box<dyn Fn() -> O>)],
+    computations: &[(&str, Box<Compute<O>>)],
 ) where
     O: PartialEq + Debug + Clone,
 {
@@ -80,7 +82,7 @@ pub fn timed_collect_all<Out, O>(
     benchmark_name: &str,
     num_repetitions: usize,
     expected_output: &[O],
-    computations: &[(&str, Box<dyn Fn() -> Out>)],
+    computations: &[(&str, Box<Compute<Out>>)],
 ) where
     Out: IntoIterator<Item = O>,
     O: PartialEq + Debug,
diff --git a/src/tests/doubly.rs b/src/tests/doubly.rs
index 8786776..efbf358 100644
--- a/src/tests/doubly.rs
+++ b/src/tests/doubly.rs
@@ -51,22 +51,22 @@ where
                 assert_ne!(self.0.ends().get(BACK_IDX), self.0.ends().get(FRONT_IDX));
 
                 let mut fwd_pointers = alloc::vec![];
-                let mut ptr = self.0.ends().get(FRONT_IDX).cloned().unwrap();
-                fwd_pointers.push(ptr.clone());
-                while let Some((next_ptr, next)) = self.next(&ptr) {
-                    assert_eq!(next.prev().get(), Some(&ptr));
+                let mut ptr = self.0.ends().get(FRONT_IDX).unwrap();
+                fwd_pointers.push(ptr);
+                while let Some((next_ptr, next)) = self.next(ptr) {
+                    assert_eq!(next.prev().get(), Some(ptr));
                     ptr = next_ptr;
-                    fwd_pointers.push(ptr.clone());
+                    fwd_pointers.push(ptr);
                 }
                 assert_eq!(fwd_pointers.len(), num_active_nodes);
 
                 let mut bwd_pointers = alloc::vec![];
-                let mut ptr = self.0.ends().get(BACK_IDX).cloned().unwrap();
-                bwd_pointers.push(ptr.clone());
-                while let Some((prev_ptr, prev)) = self.prev(&ptr) {
-                    assert_eq!(prev.next().get(), Some(&ptr));
+                let mut ptr = self.0.ends().get(BACK_IDX).unwrap();
+                bwd_pointers.push(ptr);
+                while let Some((prev_ptr, prev)) = self.prev(ptr) {
+                    assert_eq!(prev.next().get(), Some(ptr));
                     ptr = prev_ptr;
-                    bwd_pointers.push(ptr.clone());
+                    bwd_pointers.push(ptr);
                 }
 
                 bwd_pointers.reverse();
@@ -79,14 +79,12 @@ where
 
         assert_eq!(iter.next(), self.front());
 
-        let mut maybe_ptr = self.0.ends().get(FRONT_IDX).cloned();
+        let mut maybe_ptr = self.0.ends().get(FRONT_IDX);
         for _ in 1..num_active_nodes {
-            let ptr = maybe_ptr.clone().unwrap();
-            maybe_ptr = self.next(&ptr).map(|x| x.0);
+            let ptr = maybe_ptr.unwrap();
+            maybe_ptr = self.next(ptr).map(|x| x.0);
 
-            let data = maybe_ptr
-                .clone()
-                .map(|p| unsafe { self.0.data_unchecked(&p) });
+            let data = maybe_ptr.map(|p| unsafe { self.0.data_unchecked(p) });
             assert_eq!(iter.next(), data);
         }
         assert!(iter.next().is_none());
@@ -96,14 +94,12 @@ where
 
         assert_eq!(iter.next(), self.back());
 
-        let mut maybe_ptr = self.0.ends().get(BACK_IDX).cloned();
+        let mut maybe_ptr = self.0.ends().get(BACK_IDX);
         for _ in 1..num_active_nodes {
-            let ptr = maybe_ptr.clone().unwrap();
-            maybe_ptr = self.prev(&ptr).map(|x| x.0);
+            let ptr = maybe_ptr.unwrap();
+            maybe_ptr = self.prev(ptr).map(|x| x.0);
 
-            let data = maybe_ptr
-                .clone()
-                .map(|p| unsafe { self.0.data_unchecked(&p) });
+            let data = maybe_ptr.map(|p| unsafe { self.0.data_unchecked(p) });
             assert_eq!(iter.next(), data);
         }
         assert!(iter.next().is_none());
diff --git a/src/tests/singly.rs b/src/tests/singly.rs
index ddb49fd..f147dc0 100644
--- a/src/tests/singly.rs
+++ b/src/tests/singly.rs
@@ -39,11 +39,11 @@ where
                 assert!(self.front().is_some());
 
                 let mut fwd_pointers = alloc::vec![];
-                let mut ptr = self.0.ends().get().cloned().unwrap();
-                fwd_pointers.push(ptr.clone());
-                while let Some((next_ptr, _)) = self.next(&ptr) {
+                let mut ptr = self.0.ends().get().unwrap();
+                fwd_pointers.push(ptr);
+                while let Some((next_ptr, _)) = self.next(ptr) {
                     ptr = next_ptr;
-                    fwd_pointers.push(ptr.clone());
+                    fwd_pointers.push(ptr);
                 }
                 assert_eq!(fwd_pointers.len(), num_active_nodes);
             }
@@ -54,14 +54,12 @@ where
 
         assert_eq!(iter.next(), self.front());
 
-        let mut maybe_ptr = self.0.ends().get().cloned();
+        let mut maybe_ptr = self.0.ends().get();
         for _ in 1..num_active_nodes {
-            let ptr = maybe_ptr.clone().unwrap();
-            maybe_ptr = self.next(&ptr).map(|x| x.0);
+            let ptr = maybe_ptr.unwrap();
+            maybe_ptr = self.next(ptr).map(|x| x.0);
 
-            let data = maybe_ptr
-                .clone()
-                .map(|p| unsafe { self.0.data_unchecked(&p) });
+            let data = maybe_ptr.map(|p| unsafe { self.0.data_unchecked(p) });
             assert_eq!(iter.next(), data);
         }
         assert!(iter.next().is_none());
diff --git a/src/variant.rs b/src/variant.rs
index 5e72c69..c2f89ee 100644
--- a/src/variant.rs
+++ b/src/variant.rs
@@ -12,6 +12,11 @@ pub struct Singly<T> {
     p: PhantomData<T>,
 }
 
+/// # SAFETY
+///
+/// List variants do not hold any data, safe to send or sync.
+unsafe impl<T> Sync for Singly<T> {}
+
 impl<T> Variant for Singly<T> {
     type Item = T;
 
@@ -32,6 +37,11 @@ pub struct Doubly<T> {
     p: PhantomData<T>,
 }
 
+/// # SAFETY
+///
+/// List variants do not hold any data, safe to send or sync.
+unsafe impl<T> Sync for Doubly<T> {}
+
 impl<T> Variant for Doubly<T> {
     type Item = T;
 
diff --git a/tests/list_move_next_to.rs b/tests/list_move_next_to.rs
index 159bfbb..25e6850 100644
--- a/tests/list_move_next_to.rs
+++ b/tests/list_move_next_to.rs
@@ -13,7 +13,7 @@ fn list_move_next_to_front() {
         let (mut list, idx) = list_and_indices(n);
         list.move_next_to(idx[i], idx[0]);
 
-        let mut vec: Vec<_> = (0..n).into_iter().filter(|x| x != &i).collect();
+        let mut vec: Vec<_> = (0..n).filter(|x| x != &i).collect();
         vec.insert(1, i);
 
         #[cfg(feature = "validation")]
@@ -29,7 +29,7 @@ fn list_move_next_to_back() {
         let (mut list, idx) = list_and_indices(n);
         list.move_next_to(idx[i], idx[n - 1]);
 
-        let mut vec: Vec<_> = (0..n).into_iter().filter(|x| x != &i).collect();
+        let mut vec: Vec<_> = (0..n).filter(|x| x != &i).collect();
         vec.insert(n - 1, i);
 
         #[cfg(feature = "validation")]
diff --git a/tests/list_move_prev_to.rs b/tests/list_move_prev_to.rs
index de96e07..da13368 100644
--- a/tests/list_move_prev_to.rs
+++ b/tests/list_move_prev_to.rs
@@ -13,7 +13,7 @@ fn list_move_prev_to_front() {
         let (mut list, idx) = list_and_indices(n);
         list.move_prev_to(idx[i], idx[0]);
 
-        let mut vec: Vec<_> = (0..n).into_iter().filter(|x| x != &i).collect();
+        let mut vec: Vec<_> = (0..n).filter(|x| x != &i).collect();
         vec.insert(0, i);
 
         #[cfg(feature = "validation")]
@@ -29,7 +29,7 @@ fn list_move_prev_to_back() {
         let (mut list, idx) = list_and_indices(n);
         list.move_prev_to(idx[i], idx[n - 1]);
 
-        let mut vec: Vec<_> = (0..n).into_iter().filter(|x| x != &i).collect();
+        let mut vec: Vec<_> = (0..n).filter(|x| x != &i).collect();
         match i != n - 1 {
             true => vec.insert(n - 2, i),
             false => vec.push(i),
diff --git a/tests/reverse.rs b/tests/reverse.rs
index 622af03..62f3f1f 100644
--- a/tests/reverse.rs
+++ b/tests/reverse.rs
@@ -14,7 +14,7 @@ fn list_reverse() {
 #[test]
 fn slice_reverse_empty() {
     for i in 0..20 {
-        let mut list = doubly::new_doubly(&mut &mut doubly::rng_with_seed(100 * i as u64), 20, 50);
+        let mut list = doubly::new_doubly(&mut doubly::rng_with_seed(100 * i as u64), 20, 50);
         let expected: Vec<_> = list.iter().cloned().collect();
         let idx: Vec<_> = list.indices().collect();
 
@@ -30,7 +30,7 @@ fn slice_reverse_empty() {
 #[test]
 fn slice_reverse_single() {
     for i in 0..20 {
-        let mut list = doubly::new_doubly(&mut &mut doubly::rng_with_seed(100 * i as u64), 20, 50);
+        let mut list = doubly::new_doubly(&mut doubly::rng_with_seed(100 * i as u64), 20, 50);
         let expected: Vec<_> = list.iter().cloned().collect();
         let idx: Vec<_> = list.indices().collect();
 
@@ -46,7 +46,7 @@ fn slice_reverse_single() {
 #[test]
 fn slice_reverse_from_front() {
     for i in 0..20 {
-        let mut list = doubly::new_doubly(&mut &mut doubly::rng_with_seed(100 * i as u64), 20, 50);
+        let mut list = doubly::new_doubly(&mut doubly::rng_with_seed(100 * i as u64), 20, 50);
         let mut expected: Vec<_> = list.iter().cloned().collect();
         let idx: Vec<_> = list.indices().collect();
 
@@ -65,7 +65,7 @@ fn slice_reverse_from_front() {
 #[test]
 fn slice_reverse_until_back() {
     for i in 0..20 {
-        let mut list = doubly::new_doubly(&mut &mut doubly::rng_with_seed(100 * i as u64), 20, 50);
+        let mut list = doubly::new_doubly(&mut doubly::rng_with_seed(100 * i as u64), 20, 50);
         let mut expected: Vec<_> = list.iter().cloned().collect();
         let idx: Vec<_> = list.indices().collect();
 
@@ -84,7 +84,7 @@ fn slice_reverse_until_back() {
 #[test]
 fn slice_reverse_middle() {
     for i in 0..20 {
-        let mut list = doubly::new_doubly(&mut &mut doubly::rng_with_seed(100 * i as u64), 20, 50);
+        let mut list = doubly::new_doubly(&mut doubly::rng_with_seed(100 * i as u64), 20, 50);
         let mut expected: Vec<_> = list.iter().cloned().collect();
         let idx: Vec<_> = list.indices().collect();
 
diff --git a/tests/ring_iter_mut.rs b/tests/ring_iter_mut.rs
index f17b266..ba8a372 100644
--- a/tests/ring_iter_mut.rs
+++ b/tests/ring_iter_mut.rs
@@ -57,7 +57,7 @@ fn ring_iter_mut_demo() {
     fn scan<'a, I: Iterator<Item = &'a mut i32>>(mut values: I) {
         if let Some(first) = values.next() {
             let mut acc = *first;
-            while let Some(x) = values.next() {
+            for x in values {
                 let new_acc = acc + *x;
                 *x += acc;
                 acc = new_acc;
diff --git a/tests/slice_doubly.rs b/tests/slice_doubly.rs
index 18f1656..79aae9b 100644
--- a/tests/slice_doubly.rs
+++ b/tests/slice_doubly.rs
@@ -23,7 +23,7 @@ fn empty_from_nonempty() {
     let c = list.push_back('c');
     assert!(list.eq_to_iter_vals(['a', 'b', 'c']));
 
-    let indices = [a, b, c.clone()];
+    let indices = [a, b, c];
 
     for x in &indices {
         assert_empty_slice(&list.slice(x..x));
@@ -40,7 +40,7 @@ fn singleton_slice() {
     let expected = ['a', 'b', 'c'];
     assert!(list.eq_to_iter_refs(expected.iter()));
 
-    let indices = [a, b, c.clone()];
+    let indices = [a, b, c];
 
     for (i, x) in indices.iter().enumerate() {
         let slice = list.slice(x..=x);
@@ -119,8 +119,8 @@ fn doubly_slice() {
     let idx: Vec<_> = list.indices().collect();
 
     // empty
-    for i in 0..n {
-        assert_empty_slice(&list.slice(idx[i]..idx[i]));
+    for id in idx.iter().take(n) {
+        assert_empty_slice(&list.slice(id..id));
     }
 
     // single
diff --git a/tests/slice_move_next_to.rs b/tests/slice_move_next_to.rs
index 22acd48..d6e5ee9 100644
--- a/tests/slice_move_next_to.rs
+++ b/tests/slice_move_next_to.rs
@@ -23,7 +23,7 @@ fn slice_move_next_to_front() {
         #[cfg(feature = "validation")]
         list.validate();
 
-        let mut vec: Vec<_> = (0..n).into_iter().filter(|x| x != &i).collect();
+        let mut vec: Vec<_> = (0..n).filter(|x| x != &i).collect();
         vec.insert(a + 1, i);
 
         assert_eq!(slice, &vec[a..=b]);
@@ -48,7 +48,7 @@ fn slice_move_next_to_back() {
         #[cfg(feature = "validation")]
         list.validate();
 
-        let mut vec: Vec<_> = (0..n).into_iter().filter(|x| x != &i).collect();
+        let mut vec: Vec<_> = (0..n).filter(|x| x != &i).collect();
         vec.insert(b, i);
 
         assert_eq!(slice, &vec[a..=b]);
diff --git a/tests/slice_move_prev_to.rs b/tests/slice_move_prev_to.rs
index 72470d0..5cee7ec 100644
--- a/tests/slice_move_prev_to.rs
+++ b/tests/slice_move_prev_to.rs
@@ -23,7 +23,7 @@ fn slice_move_prev_to_front() {
         #[cfg(feature = "validation")]
         list.validate();
 
-        let mut vec: Vec<_> = (0..n).into_iter().filter(|x| x != &i).collect();
+        let mut vec: Vec<_> = (0..n).filter(|x| x != &i).collect();
         vec.insert(a, i);
 
         assert_eq!(slice, &vec[a..=b]);
@@ -48,7 +48,7 @@ fn slice_move_prev_to_back() {
         #[cfg(feature = "validation")]
         list.validate();
 
-        let mut vec: Vec<_> = (0..n).into_iter().filter(|x| x != &i).collect();
+        let mut vec: Vec<_> = (0..n).filter(|x| x != &i).collect();
         match i != b {
             true => vec.insert(b - 1, i),
             false => vec.insert(b, i),
diff --git a/tests/slice_mut_doubly.rs b/tests/slice_mut_doubly.rs
index c3dc214..c1b7012 100644
--- a/tests/slice_mut_doubly.rs
+++ b/tests/slice_mut_doubly.rs
@@ -23,7 +23,7 @@ fn empty_from_nonempty() {
     let c = list.push_back('c');
     assert!(list.eq_to_iter_vals(['a', 'b', 'c']));
 
-    let indices = [a, b, c.clone()];
+    let indices = [a, b, c];
 
     for x in &indices {
         assert_empty_slice(&list.slice_mut(x..x));
@@ -39,7 +39,7 @@ fn singleton_slice() {
 
     let expected = ['x', 'y', 'z'];
 
-    let indices = [a, b, c.clone()];
+    let indices = [a, b, c];
 
     for (i, x) in indices.iter().copied().enumerate() {
         let mut slice = list.slice_mut(x..=x);