diff --git a/src/flow/traversible.rs b/src/flow/traversible.rs
index 357e7e2..5be1623 100644
--- a/src/flow/traversible.rs
+++ b/src/flow/traversible.rs
@@ -1,5 +1,6 @@
 //! Traversible binary trees.
 
+pub mod algorithms;
 pub mod unbalanced;
 
 use std::rc::Rc;
@@ -66,127 +67,3 @@ pub trait TraversibleBinaryReference<'a, T: 'a + Monad, A: 'a, D: 'a + PartialEq
 pub trait TraversibleBinaryTree<'a, T: 'a + Monad, A: 'a, D: 'a + PartialEq>: 'a {
     fn refer(&self) -> Option<Rc<dyn TraversibleBinaryReference<'a, T, A, D>>>;
 }
-
-pub fn n_contains<'a, T: 'a + MonadFail<()>, A: 'a, D: 'a + PartialEq>(
-    comparator: &'a dyn Comparator<A>,
-    n_set: Rc<dyn TraversibleBinaryNode<'a, T, A, D>>,
-    key: Rc<A>,
-) -> T::F<'a, ()> {
-    let (t_setl, t_setr, k_set) = n_set.split();
-    match comparator.pick_smaller(key.as_ref(), k_set.as_ref()) {
-        Comparison::L => t_contains(comparator, t_setl, key),
-        Comparison::E => T::pure(()),
-        Comparison::R => t_contains(comparator, t_setr, key),
-    }
-}
-
-pub fn r_contains<'a, T: 'a + MonadFail<()>, A: 'a, D: 'a + PartialEq>(
-    comparator: &'a dyn Comparator<A>,
-    r_set: Rc<dyn TraversibleBinaryReference<'a, T, A, D>>,
-    key: Rc<A>,
-) -> T::F<'a, ()> {
-    T::bind(r_set.resolve(), |n_set| n_contains(comparator, n_set, key))
-}
-
-pub fn t_contains<'a, T: 'a + MonadFail<()>, A: 'a, D: 'a + PartialEq>(
-    comparator: &'a dyn Comparator<A>,
-    t_set: Rc<dyn TraversibleBinaryTree<'a, T, A, D>>,
-    key: Rc<A>,
-) -> T::F<'a, ()> {
-    match t_set.refer() {
-        Some(r_set) => r_contains(comparator, r_set, key),
-        None => T::fail(()),
-    }
-}
-
-pub fn n_subset_of_n<'a, T: 'a + MonadFail<()>, A: 'a, D: 'a + PartialEq>(
-    comparator: &'a dyn Comparator<A>,
-    n_subset: Rc<dyn TraversibleBinaryNode<'a, T, A, D>>,
-    n_superset: Rc<dyn TraversibleBinaryNode<'a, T, A, D>>,
-    k_l: Option<Rc<A>>,
-    k_r: Option<Rc<A>>,
-) -> T::F<'a, ()> {
-    let (t_superl, t_superr, k_super) = n_superset.split();
-    if let Some(ref a_l) = k_l {
-        if let Comparison::R = comparator.pick_smaller(a_l.as_ref(), k_super.as_ref()) {
-            return t_subset_of_t(comparator, n_subset.to_tree(), t_superr, k_l, k_r);
-        }
-    }
-    if let Some(ref a_r) = k_r {
-        if let Comparison::L = comparator.pick_smaller(a_r.as_ref(), k_super.as_ref()) {
-            return t_subset_of_t(comparator, n_subset.to_tree(), t_superl, k_l, k_r);
-        }
-    }
-    let (t_subl, t_subr, k_sub) = n_subset.split();
-    match comparator.pick_smaller(k_sub.as_ref(), k_super.as_ref()) {
-        Comparison::L => T::la2(
-            and,
-            t_contains(comparator, t_superl.clone(), k_sub.clone()),
-            T::la2(
-                and,
-                t_subset_of_t(comparator, t_subl, t_superl, k_l, Some(k_sub.clone())),
-                t_subset_of_t(
-                    comparator,
-                    t_subr,
-                    n_superset.to_tree(),
-                    Some(k_sub.clone()),
-                    k_r,
-                ),
-            ),
-        ),
-        Comparison::E => T::la2(
-            and,
-            t_subset_of_t(comparator, t_subl, t_superl, k_l, Some(k_sub.clone())),
-            t_subset_of_t(comparator, t_subr, t_superr, Some(k_sub.clone()), k_r),
-        ),
-        Comparison::R => T::la2(
-            and,
-            t_contains(comparator, t_superr.clone(), k_sub.clone()),
-            T::la2(
-                and,
-                t_subset_of_t(
-                    comparator,
-                    t_subl,
-                    n_superset.to_tree(),
-                    k_l,
-                    Some(k_sub.clone()),
-                ),
-                t_subset_of_t(comparator, t_subr, t_superr, Some(k_sub.clone()), k_r),
-            ),
-        ),
-    }
-}
-
-pub fn r_subset_of_r<'a, T: 'a + MonadFail<()>, A: 'a, D: 'a + PartialEq>(
-    comparator: &'a dyn Comparator<A>,
-    r_subset: Rc<dyn TraversibleBinaryReference<'a, T, A, D>>,
-    r_superset: Rc<dyn TraversibleBinaryReference<'a, T, A, D>>,
-    k_l: Option<Rc<A>>,
-    k_r: Option<Rc<A>>,
-) -> T::F<'a, ()> {
-    T::join(T::la2(
-        move |n_subset, n_superset| n_subset_of_n(comparator, n_subset, n_superset, k_l, k_r),
-        r_subset.resolve(),
-        r_superset.resolve(),
-    ))
-}
-
-pub fn t_subset_of_t<'a, T: MonadFail<()>, A, D: 'a + PartialEq>(
-    comparator: &'a dyn Comparator<A>,
-    t_subset: Rc<dyn TraversibleBinaryTree<'a, T, A, D>>,
-    t_superset: Rc<dyn TraversibleBinaryTree<'a, T, A, D>>,
-    k_l: Option<Rc<A>>,
-    k_r: Option<Rc<A>>,
-) -> T::F<'a, ()> {
-    match (t_subset.refer(), t_superset.refer()) {
-        (None, _) => T::pure(()),
-        (Some(_), None) => T::fail(()),
-        (Some(r_subset), Some(r_superset)) => {
-            if r_subset.data() == r_superset.data() {
-                T::pure(())
-            } else {
-                r_subset_of_r(comparator, r_subset, r_superset, k_l, k_r)
-            }
-        }
-    }
-}
diff --git a/src/flow/traversible/algorithms.rs b/src/flow/traversible/algorithms.rs
new file mode 100644
index 0000000..5b12cee
--- /dev/null
+++ b/src/flow/traversible/algorithms.rs
@@ -0,0 +1,2 @@
+pub mod contains;
+pub mod subset;
diff --git a/src/flow/traversible/algorithms/contains.rs b/src/flow/traversible/algorithms/contains.rs
new file mode 100644
index 0000000..4b299ea
--- /dev/null
+++ b/src/flow/traversible/algorithms/contains.rs
@@ -0,0 +1,33 @@
+use crate::flow::traversible::*;
+
+pub fn n_contains<'a, T: 'a + MonadFail<()>, A: 'a, D: 'a + PartialEq>(
+    comparator: &'a dyn Comparator<A>,
+    n_set: Rc<dyn TraversibleBinaryNode<'a, T, A, D>>,
+    key: Rc<A>,
+) -> T::F<'a, ()> {
+    let (t_setl, t_setr, k_set) = n_set.split();
+    match comparator.pick_smaller(key.as_ref(), k_set.as_ref()) {
+        Comparison::L => t_contains(comparator, t_setl, key),
+        Comparison::E => T::pure(()),
+        Comparison::R => t_contains(comparator, t_setr, key),
+    }
+}
+
+pub fn r_contains<'a, T: 'a + MonadFail<()>, A: 'a, D: 'a + PartialEq>(
+    comparator: &'a dyn Comparator<A>,
+    r_set: Rc<dyn TraversibleBinaryReference<'a, T, A, D>>,
+    key: Rc<A>,
+) -> T::F<'a, ()> {
+    T::bind(r_set.resolve(), |n_set| n_contains(comparator, n_set, key))
+}
+
+pub fn t_contains<'a, T: 'a + MonadFail<()>, A: 'a, D: 'a + PartialEq>(
+    comparator: &'a dyn Comparator<A>,
+    t_set: Rc<dyn TraversibleBinaryTree<'a, T, A, D>>,
+    key: Rc<A>,
+) -> T::F<'a, ()> {
+    match t_set.refer() {
+        Some(r_set) => r_contains(comparator, r_set, key),
+        None => T::fail(()),
+    }
+}
diff --git a/src/flow/traversible/algorithms/subset.rs b/src/flow/traversible/algorithms/subset.rs
new file mode 100644
index 0000000..1066782
--- /dev/null
+++ b/src/flow/traversible/algorithms/subset.rs
@@ -0,0 +1,159 @@
+use crate::flow::traversible::*;
+
+struct SubsetContext<'a, T: 'a + MonadFail<()>, A: 'a, D: 'a + PartialEq> {
+    comparator: &'a dyn Comparator<A>,
+    n_subset: Rc<dyn TraversibleBinaryNode<'a, T, A, D>>,
+    n_superset: Rc<dyn TraversibleBinaryNode<'a, T, A, D>>,
+    k_l: Option<Rc<A>>,
+    k_r: Option<Rc<A>>,
+    t_superl: Rc<dyn TraversibleBinaryTree<'a, T, A, D>>,
+    t_superr: Rc<dyn TraversibleBinaryTree<'a, T, A, D>>,
+    k_super: Rc<A>,
+}
+
+impl<'a, T: 'a + MonadFail<()>, A: 'a, D: 'a + PartialEq> SubsetContext<'a, T, A, D> {
+    fn on_l(self, (t_subl, t_subr, k_sub): Split<'a, T, A, D>) -> T::F<'a, ()> {
+        T::la2(
+            and,
+            algorithms::contains::t_contains(self.comparator, self.t_superl.clone(), k_sub.clone()),
+            T::la2(
+                and,
+                t_subset_of_t(
+                    self.comparator,
+                    t_subl,
+                    self.t_superl,
+                    self.k_l,
+                    Some(k_sub.clone()),
+                ),
+                t_subset_of_t(
+                    self.comparator,
+                    t_subr,
+                    self.n_superset.to_tree(),
+                    Some(k_sub.clone()),
+                    self.k_r,
+                ),
+            ),
+        )
+    }
+
+    fn on_e(self, (t_subl, t_subr, k_sub): Split<'a, T, A, D>) -> T::F<'a, ()> {
+        T::la2(
+            and,
+            t_subset_of_t(
+                self.comparator,
+                t_subl,
+                self.t_superl,
+                self.k_l,
+                Some(k_sub.clone()),
+            ),
+            t_subset_of_t(
+                self.comparator,
+                t_subr,
+                self.t_superr,
+                Some(k_sub.clone()),
+                self.k_r,
+            ),
+        )
+    }
+
+    fn on_r(self, (t_subl, t_subr, k_sub): Split<'a, T, A, D>) -> T::F<'a, ()> {
+        T::la2(
+            and,
+            algorithms::contains::t_contains(self.comparator, self.t_superr.clone(), k_sub.clone()),
+            T::la2(
+                and,
+                t_subset_of_t(
+                    self.comparator,
+                    t_subl,
+                    self.n_superset.to_tree(),
+                    self.k_l,
+                    Some(k_sub.clone()),
+                ),
+                t_subset_of_t(
+                    self.comparator,
+                    t_subr,
+                    self.t_superr,
+                    Some(k_sub.clone()),
+                    self.k_r,
+                ),
+            ),
+        )
+    }
+
+    fn test(self) -> T::F<'a, ()> {
+        let split = self.n_subset.split();
+        match self
+            .comparator
+            .pick_smaller(split.2.as_ref(), self.k_super.as_ref())
+        {
+            Comparison::L => self.on_l(split),
+            Comparison::E => self.on_e(split),
+            Comparison::R => self.on_r(split),
+        }
+    }
+}
+
+pub fn n_subset_of_n<'a, T: 'a + MonadFail<()>, A: 'a, D: 'a + PartialEq>(
+    comparator: &'a dyn Comparator<A>,
+    n_subset: Rc<dyn TraversibleBinaryNode<'a, T, A, D>>,
+    n_superset: Rc<dyn TraversibleBinaryNode<'a, T, A, D>>,
+    k_l: Option<Rc<A>>,
+    k_r: Option<Rc<A>>,
+) -> T::F<'a, ()> {
+    let (t_superl, t_superr, k_super) = n_superset.split();
+    if let Some(ref a_l) = k_l {
+        if let Comparison::R = comparator.pick_smaller(a_l.as_ref(), k_super.as_ref()) {
+            return t_subset_of_t(comparator, n_subset.to_tree(), t_superr, k_l, k_r);
+        }
+    }
+    if let Some(ref a_r) = k_r {
+        if let Comparison::L = comparator.pick_smaller(a_r.as_ref(), k_super.as_ref()) {
+            return t_subset_of_t(comparator, n_subset.to_tree(), t_superl, k_l, k_r);
+        }
+    }
+    SubsetContext {
+        comparator,
+        n_subset,
+        n_superset,
+        k_l,
+        k_r,
+        t_superl,
+        t_superr,
+        k_super,
+    }
+    .test()
+}
+
+pub fn r_subset_of_r<'a, T: 'a + MonadFail<()>, A: 'a, D: 'a + PartialEq>(
+    comparator: &'a dyn Comparator<A>,
+    r_subset: Rc<dyn TraversibleBinaryReference<'a, T, A, D>>,
+    r_superset: Rc<dyn TraversibleBinaryReference<'a, T, A, D>>,
+    k_l: Option<Rc<A>>,
+    k_r: Option<Rc<A>>,
+) -> T::F<'a, ()> {
+    T::join(T::la2(
+        move |n_subset, n_superset| n_subset_of_n(comparator, n_subset, n_superset, k_l, k_r),
+        r_subset.resolve(),
+        r_superset.resolve(),
+    ))
+}
+
+pub fn t_subset_of_t<'a, T: MonadFail<()>, A, D: 'a + PartialEq>(
+    comparator: &'a dyn Comparator<A>,
+    t_subset: Rc<dyn TraversibleBinaryTree<'a, T, A, D>>,
+    t_superset: Rc<dyn TraversibleBinaryTree<'a, T, A, D>>,
+    k_l: Option<Rc<A>>,
+    k_r: Option<Rc<A>>,
+) -> T::F<'a, ()> {
+    match (t_subset.refer(), t_superset.refer()) {
+        (None, _) => T::pure(()),
+        (Some(_), None) => T::fail(()),
+        (Some(r_subset), Some(r_superset)) => {
+            if r_subset.data() == r_superset.data() {
+                T::pure(())
+            } else {
+                r_subset_of_r(comparator, r_subset, r_superset, k_l, k_r)
+            }
+        }
+    }
+}
diff --git a/src/flow/traversible/unbalanced.rs b/src/flow/traversible/unbalanced.rs
index aab785c..9b58b5a 100644
--- a/src/flow/traversible/unbalanced.rs
+++ b/src/flow/traversible/unbalanced.rs
@@ -175,7 +175,7 @@ mod tests {
 
     use crate::std::tracing::*;
 
-    use super::*;
+    use super::{algorithms::contains::*, algorithms::subset::*, *};
 
     #[test]
     fn test_simple_slices() {