diff --git a/src/flow/binary/bounds.rs b/src/flow/binary/bounds.rs
index 304f884..1f7917d 100644
--- a/src/flow/binary/bounds.rs
+++ b/src/flow/binary/bounds.rs
@@ -1,3 +1,5 @@
+pub mod bound;
+
 use crate::flow::comparator::*;
 
 #[derive(Clone)]
diff --git a/src/flow/binary/bounds/bound.rs b/src/flow/binary/bounds/bound.rs
new file mode 100644
index 0000000..a3909ea
--- /dev/null
+++ b/src/flow/binary/bounds/bound.rs
@@ -0,0 +1,99 @@
+use crate::{flow::binary::*, func::context::*};
+
+use super::*;
+
+#[derive(Clone)]
+pub struct Bound<A, T> {
+    bound: T,
+    bounds: Bounds<A>,
+}
+
+pub struct BoundNode2<A, T> {
+    boundsl: Bounds<A>,
+    boundsr: Bounds<A>,
+    bounds: Bounds<A>,
+    node: T,
+}
+
+#[derive(Clone)]
+pub struct BoundTrees<BT>(BT);
+
+impl<'a, BT: FunctorContext<'a>> FunctorContext<'a> for BoundTrees<BT> {
+    type T = BT::T;
+}
+
+trait BinaryTreesBindable<'a>: BinaryTrees<'a> {
+    fn bounds_error<T: 'a>(&self, error: BoundsError<Self::Key>) -> BTWrap<'a, Self, T>;
+}
+
+impl<'a, BT: BinaryTreesBindable<'a>> BinaryTrees<'a> for BoundTrees<BT>
+where
+    BT::Tree: Clone,
+{
+    type Node = BoundNode2<Self::Key, BT::Node>;
+
+    type Reference = Bound<Self::Key, BT::Reference>;
+
+    type Tree = Bound<Self::Key, BT::Tree>;
+
+    type Key = BT::Key;
+
+    type Comparator = BT::Comparator;
+
+    type _Tm = Self::T;
+
+    fn comparator(&self) -> &Self::Comparator {
+        self.0.comparator()
+    }
+
+    fn split(&self, node: &Self::Node) -> Split<'a, Self> {
+        let (tl, tr, key) = self.0.split(&node.node);
+        (
+            Bound {
+                bound: tl,
+                bounds: node.boundsl.clone(),
+            },
+            Bound {
+                bound: tr,
+                bounds: node.boundsr.clone(),
+            },
+            key,
+        )
+    }
+
+    fn tree_of(&self, node: Self::Node) -> BTWrap<'a, Self, Self::Tree> {
+        Self::fmap(self.0.tree_of(node.node), move |tree| Bound {
+            bound: tree,
+            bounds: node.bounds,
+        })
+    }
+
+    fn resolve(&self, reference: &Self::Reference) -> BTWrap<'a, Self, Self::Node> {
+        let ctx = self.clone();
+        let bounds = reference.bounds.clone();
+        Self::bind(self.0.resolve(&reference.bound), move |node| {
+            let (_, _, key) = ctx.0.split(&node);
+            match bounds.clone().split(&key, ctx.comparator()) {
+                Ok((boundsl, boundsr)) => Self::pure(BoundNode2 {
+                    boundsl,
+                    boundsr,
+                    bounds,
+                    node,
+                }),
+                Err(e) => ctx.0.bounds_error(e),
+            }
+        })
+    }
+
+    fn equal(&self, rl: &Self::Reference, rr: &Self::Reference) -> bool {
+        Bounds::equal(&rl.bounds, &rr.bounds, self.comparator())
+            && self.0.equal(&rl.bound, &rr.bound)
+    }
+
+    fn refer(&self, tree: &Self::Tree) -> Option<Self::Reference> {
+        Some(Bound {
+            bound: self.0.refer(&tree.bound)?,
+            bounds: tree.bounds.clone(),
+        })
+    }
+}