diff --git a/src/flow/binary.rs b/src/flow/binary.rs
index adde5ff..d3a18cb 100644
--- a/src/flow/binary.rs
+++ b/src/flow/binary.rs
@@ -34,9 +34,17 @@ pub trait BinaryTrees<'a>: FunctorContext<'a, T = Self::_Tm> + Clone {
     }
 }
 
-pub trait BinaryTreesMutable<'a>: BinaryTrees<'a> {
+pub trait BinaryTreesEmpty<'a>: BinaryTrees<'a> {
     fn empty(&self) -> Self::Tree;
 
+    fn split_key_empty(
+        &self,
+        tree: Self::Tree,
+        key: Self::Key,
+    ) -> BTWrap<'a, Self, KeySplit<'a, Self>>;
+}
+
+pub trait BinaryTreesMutable<'a>: BinaryTreesEmpty<'a> {
     fn join_key(
         self,
         tl: Self::Tree,
@@ -76,12 +84,6 @@ pub trait BinaryTreesMutable<'a>: BinaryTrees<'a> {
         })
     }
 
-    fn split_key_empty(
-        &self,
-        tree: Self::Tree,
-        key: Self::Key,
-    ) -> BTWrap<'a, Self, KeySplit<'a, Self>>;
-
     fn split_key_node(
         self,
         node: Self::Node,
diff --git a/src/flow/binary/avl.rs b/src/flow/binary/avl.rs
index 0670839..c51b841 100644
--- a/src/flow/binary/avl.rs
+++ b/src/flow/binary/avl.rs
@@ -146,7 +146,7 @@ impl<'a, A: 'a + PartialOrd + Clone> BinaryTrees<'a> for AvlTs<A> {
     }
 }
 
-impl<'a, A: 'a + PartialOrd + Clone> BinaryTreesMutable<'a> for AvlTs<A> {
+impl<'a, A: 'a + PartialOrd + Clone> BinaryTreesEmpty<'a> for AvlTs<A> {
     fn empty(&self) -> Self::Tree {
         AvlT {
             reference: None,
@@ -154,15 +154,6 @@ impl<'a, A: 'a + PartialOrd + Clone> BinaryTreesMutable<'a> for AvlTs<A> {
         }
     }
 
-    fn join_key(
-        self,
-        tl: Self::Tree,
-        key: Self::Key,
-        tr: Self::Tree,
-    ) -> BTWrap<'a, Self, Self::Node> {
-        self.join_key_balanced(tl, key, tr)
-    }
-
     fn split_key_empty(
         &self,
         _tree: Self::Tree,
@@ -172,6 +163,17 @@ impl<'a, A: 'a + PartialOrd + Clone> BinaryTreesMutable<'a> for AvlTs<A> {
     }
 }
 
+impl<'a, A: 'a + PartialOrd + Clone> BinaryTreesMutable<'a> for AvlTs<A> {
+    fn join_key(
+        self,
+        tl: Self::Tree,
+        key: Self::Key,
+        tr: Self::Tree,
+    ) -> BTWrap<'a, Self, Self::Node> {
+        self.join_key_balanced(tl, key, tr)
+    }
+}
+
 impl<'a, A: 'a + PartialOrd + Clone> BinaryTreesAvl<'a> for AvlTs<A> {
     fn height(&self, tree: &Self::Tree) -> u64 {
         tree.height
diff --git a/src/flow/binary/bounds/bound.rs b/src/flow/binary/bounds/bound.rs
index 958402a..5c71d95 100644
--- a/src/flow/binary/bounds/bound.rs
+++ b/src/flow/binary/bounds/bound.rs
@@ -94,3 +94,38 @@ impl<'a, BT: BinaryTreesBindable<'a>> BinaryTrees<'a> for BoundTrees<BT> {
         })
     }
 }
+
+impl<'a, BT: BinaryTreesBindable<'a> + BinaryTreesEmpty<'a>> BinaryTreesEmpty<'a>
+    for BoundTrees<BT>
+{
+    fn empty(&self) -> Self::Tree {
+        Bound {
+            bound: self.0.empty(),
+            bounds: Bounds::unbound(),
+        }
+    }
+
+    fn split_key_empty(
+        &self,
+        tree: Self::Tree,
+        key: Self::Key,
+    ) -> BTWrap<'a, Self, KeySplit<'a, Self>> {
+        match tree.bounds.split(&key, self.comparator()) {
+            Ok((boundsl, boundsr)) => {
+                Self::bind(self.0.split_key_empty(tree.bound, key), |(tl, tr)| {
+                    Self::pure((
+                        Bound {
+                            bound: tl,
+                            bounds: boundsl,
+                        },
+                        Bound {
+                            bound: tr,
+                            bounds: boundsr,
+                        },
+                    ))
+                })
+            }
+            Err(e) => self.0.bounds_error(e),
+        }
+    }
+}
diff --git a/src/rstd/collections/avl/context.rs b/src/rstd/collections/avl/context.rs
index c8d3b17..27ea2d7 100644
--- a/src/rstd/collections/avl/context.rs
+++ b/src/rstd/collections/avl/context.rs
@@ -67,21 +67,12 @@ impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx> + Clone, C: 'a + Comparator<A
 }
 
 impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx> + Clone, C: 'a + Comparator<A>>
-    BinaryTreesMutable<'a> for BoundContext<'a, Ctx, A, C>
+    BinaryTreesEmpty<'a> for BoundContext<'a, Ctx, A, C>
 {
     fn empty(&self) -> Self::Tree {
         BoundTree::empty(self.factory.clone())
     }
 
-    fn join_key(
-        self,
-        tl: Self::Tree,
-        key: Self::Key,
-        tr: Self::Tree,
-    ) -> BTWrap<'a, Self, Self::Node> {
-        self.join_key_balanced(tl, key, tr)
-    }
-
     fn split_key_empty(
         &self,
         tree: Self::Tree,
@@ -91,6 +82,19 @@ impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx> + Clone, C: 'a + Comparator<A
     }
 }
 
+impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx> + Clone, C: 'a + Comparator<A>>
+    BinaryTreesMutable<'a> for BoundContext<'a, Ctx, A, C>
+{
+    fn join_key(
+        self,
+        tl: Self::Tree,
+        key: Self::Key,
+        tr: Self::Tree,
+    ) -> BTWrap<'a, Self, Self::Node> {
+        self.join_key_balanced(tl, key, tr)
+    }
+}
+
 impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx> + Clone, C: 'a + Comparator<A>>
     BinaryTreesAvl<'a> for BoundContext<'a, Ctx, A, C>
 {