diff --git a/book-radn b/book-radn
index 343baf1..bb44e06 160000
--- a/book-radn
+++ b/book-radn
@@ -1 +1 @@
-Subproject commit 343baf150fe86ad6cd3594828068d42e7bc25b18
+Subproject commit bb44e06dea5b3a00585066063be00c4b3a976f70
diff --git a/src/flow.rs b/src/flow.rs
index 60bd286..0d4dc51 100644
--- a/src/flow.rs
+++ b/src/flow.rs
@@ -1,3 +1,4 @@
 //! Data structures and algorithms, independent of [`crate::core`] concepts.
 
+pub mod binary;
 pub mod traversible;
diff --git a/src/flow/binary.rs b/src/flow/binary.rs
new file mode 100644
index 0000000..946c59f
--- /dev/null
+++ b/src/flow/binary.rs
@@ -0,0 +1,30 @@
+use std::rc::Rc;
+
+use crate::func::*;
+
+pub type NodeRc<'a, BT> = Rc<<BT as BinaryTrees<'a>>::Node>;
+pub type ReferenceRc<'a, BT> = Rc<<BT as BinaryTrees<'a>>::Reference>;
+pub type TreeRc<'a, BT> = Rc<<BT as BinaryTrees<'a>>::Tree>;
+pub type KeyRc<'a, BT> = Rc<<BT as BinaryTrees<'a>>::Key>;
+
+pub type Split<'a, BT> = (TreeRc<'a, BT>, TreeRc<'a, BT>, KeyRc<'a, BT>);
+
+pub type Wrapped<'a, BT, A> = <<BT as BinaryTrees<'a>>::T as WeakFunctor>::F<'a, A>;
+
+pub trait BinaryTrees<'a>: 'a {
+    type Node: 'a;
+    type Reference: 'a;
+    type Tree: 'a;
+    type Key: 'a;
+
+    type T: 'a + Monad;
+
+    fn split(node: Self::Node) -> Wrapped<'a, Self, Split<'a, Self>>;
+    fn to_tree(node: Self::Node) -> TreeRc<'a, Self>;
+    fn to_tree_construct(&self, node: Self::Node) -> TreeRc<'a, Self> {
+        Self::to_tree(node)
+    }
+    fn resolve(reference: Self::Reference) -> Wrapped<'a, Self, NodeRc<'a, Self>>;
+    fn equal(rhs: Self::Reference, lhs: Self::Reference) -> bool;
+    fn refer(tree: Self::Tree) -> Option<ReferenceRc<'a, Self>>;
+}