diff --git a/src/flow.rs b/src/flow.rs
index 21eae0c..81040b7 100644
--- a/src/flow.rs
+++ b/src/flow.rs
@@ -1,5 +1,6 @@
 //! Data structures and algorithms, independent of [`crate::rcore`] concepts.
 
 pub mod binary;
+pub mod comparator;
 pub mod speculative;
 pub mod traversible;
diff --git a/src/flow/comparator.rs b/src/flow/comparator.rs
new file mode 100644
index 0000000..e398914
--- /dev/null
+++ b/src/flow/comparator.rs
@@ -0,0 +1,35 @@
+/// Result returned by [`Comparator::pick_smaller`].
+#[derive(Debug, PartialEq)]
+pub enum Comparison {
+    L,
+    /// Compared values were equal.
+    E,
+    R,
+}
+
+/// Returns [`Comparison`] saying which value is smaller.
+///
+/// ```rust
+/// # use radn_rs::flow::traversible::*;
+/// assert_eq!(DefaultComparator.pick_smaller(&1, &3), Comparison::L);
+/// assert_eq!(DefaultComparator.pick_smaller(&2, &2), Comparison::E);
+/// assert_eq!(DefaultComparator.pick_smaller(&3, &1), Comparison::R);
+/// ```
+pub trait Comparator<A> {
+    fn pick_smaller(&self, kl: &A, kr: &A) -> Comparison;
+}
+
+/// Implementation of a [Comparator] relying on [`PartialOrd`].
+pub struct DefaultComparator;
+
+impl<A: PartialOrd> Comparator<A> for DefaultComparator {
+    fn pick_smaller(&self, kl: &A, kr: &A) -> Comparison {
+        if kl < kr {
+            Comparison::L
+        } else if kr < kl {
+            Comparison::R
+        } else {
+            Comparison::E
+        }
+    }
+}
diff --git a/src/flow/traversible.rs b/src/flow/traversible.rs
index 1e84cc4..dd16dec 100644
--- a/src/flow/traversible.rs
+++ b/src/flow/traversible.rs
@@ -5,46 +5,9 @@ pub mod unbalanced;
 
 use std::rc::Rc;
 
+use crate::flow::comparator::*;
 use crate::func::*;
 
-/// Result returned by [`Comparator::pick_smaller`].
-#[derive(Debug, PartialEq)]
-pub enum Comparison {
-    L,
-    /// Compared values were equal.
-    E,
-    R,
-}
-
-fn and(_l: (), _r: ()) {}
-
-/// Returns [`Comparison`] saying which value is smaller.
-///
-/// ```rust
-/// # use radn_rs::flow::traversible::*;
-/// assert_eq!(DefaultComparator.pick_smaller(&1, &3), Comparison::L);
-/// assert_eq!(DefaultComparator.pick_smaller(&2, &2), Comparison::E);
-/// assert_eq!(DefaultComparator.pick_smaller(&3, &1), Comparison::R);
-/// ```
-pub trait Comparator<A> {
-    fn pick_smaller(&self, kl: &A, kr: &A) -> Comparison;
-}
-
-/// Implementation of a [Comparator] relying on [`PartialOrd`].
-pub struct DefaultComparator;
-
-impl<A: PartialOrd> Comparator<A> for DefaultComparator {
-    fn pick_smaller(&self, kl: &A, kr: &A) -> Comparison {
-        if kl < kr {
-            Comparison::L
-        } else if kr < kl {
-            Comparison::R
-        } else {
-            Comparison::E
-        }
-    }
-}
-
 pub type Split<'a, T, A, D> = (
     Rc<dyn TraversibleBinaryTree<'a, T, A, D>>,
     Rc<dyn TraversibleBinaryTree<'a, T, A, D>>,
diff --git a/src/flow/traversible/algorithms/subset.rs b/src/flow/traversible/algorithms/subset.rs
index 76d9e81..381ba49 100644
--- a/src/flow/traversible/algorithms/subset.rs
+++ b/src/flow/traversible/algorithms/subset.rs
@@ -1,5 +1,8 @@
+use crate::flow::comparator::*;
 use crate::flow::traversible::*;
 
+fn and(_l: (), _r: ()) {}
+
 struct SubsetContext<'a, T: MonadFail<'a, ()>, A: 'a, D: 'a + PartialEq> {
     comparator: &'a dyn Comparator<A>,
     n_subset: Rc<dyn TraversibleBinaryNode<'a, T, A, D>>,