diff --git a/src/flow/binary.rs b/src/flow/binary.rs
index 726f516..d850223 100644
--- a/src/flow/binary.rs
+++ b/src/flow/binary.rs
@@ -1,20 +1,21 @@
+mod avl;
+
 use std::rc::Rc;
 
-use crate::func::*;
+use crate::{flow::comparator::*, func::*};
 
-use super::comparator::Comparator;
-
-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 KeySplit<'a, BT> = (TreeRc<'a, BT>, TreeRc<'a, BT>);
+pub type Split<'a, BT> = (
+    <BT as BinaryTrees<'a>>::Tree,
+    <BT as BinaryTrees<'a>>::Tree,
+    KeyRc<'a, BT>,
+);
+pub type KeySplit<'a, BT> = (<BT as BinaryTrees<'a>>::Tree, <BT as BinaryTrees<'a>>::Tree);
 
 pub type BTWrap<'a, BT, A> = Wrap<'a, A, <BT as BinaryTrees<'a>>::T>;
 
-pub trait BinaryTrees<'a>: 'a {
+pub trait BinaryTrees<'a>: 'a + Clone {
     type Node: 'a;
     type Reference: 'a;
     type Tree: 'a;
@@ -25,24 +26,188 @@ pub trait BinaryTrees<'a>: 'a {
 
     fn comparator(&self) -> &Self::Comparator;
     fn split(&self, node: &Self::Node) -> Split<'a, Self>;
-    fn tree_of(&self, node: Self::Node) -> BTWrap<'a, Self, TreeRc<'a, Self>>;
-    fn resolve(&self, reference: &Self::Reference) -> BTWrap<'a, Self, NodeRc<'a, Self>>;
+    fn tree_of(&self, node: Self::Node) -> BTWrap<'a, Self, Self::Tree>;
+    fn resolve(&self, reference: &Self::Reference) -> BTWrap<'a, Self, Self::Node>;
     fn equal(&self, rhs: &Self::Reference, lhs: &Self::Reference) -> bool;
-    fn refer(&self, tree: &Self::Tree) -> Option<ReferenceRc<'a, Self>>;
+    fn refer(&self, tree: &Self::Tree) -> Option<Self::Reference>;
 }
 
 pub trait BinaryTreesMutable<'a>: BinaryTrees<'a> {
     fn empty(&self) -> Self::Tree;
+
     fn join_key(
-        &self,
+        self,
         tl: Self::Tree,
         key: KeyRc<'a, Self>,
         tr: Self::Tree,
-    ) -> BTWrap<'a, Self, NodeRc<'a, Self>>;
-    fn join(&self, tl: Self::Tree, tr: Self::Tree) -> BTWrap<'a, Self, TreeRc<'a, Self>>;
-    fn split_key(
+    ) -> BTWrap<'a, Self, Self::Node>;
+
+    fn join_key_tree(
+        self,
+        tl: Self::Tree,
+        key: KeyRc<'a, Self>,
+        tr: Self::Tree,
+    ) -> BTWrap<'a, Self, Self::Tree> {
+        <Self::T as Monad>::bind(self.clone().join_key(tl, key, tr), move |n| self.tree_of(n))
+    }
+
+    fn join(&self, tl: Self::Tree, tr: Self::Tree) -> BTWrap<'a, Self, Self::Tree> {
+        let Some(rl) = self.refer(&tl) else {
+            return <Self::T as Pure>::pure(tr)
+        };
+        let Some(rr) = self.refer(&tr) else {
+            return <Self::T as Pure>::pure(tl)
+        };
+        let ctx = self.clone();
+        <Self::T as Monad>::join(<Self::T as ApplicativeLA2>::la2(
+            move |nl, nr| {
+                let (tll, tlr, kl) = ctx.split(&nl);
+                let (trl, trr, kr) = ctx.split(&nr);
+                let ft = ctx.join(tlr, trl);
+                let ft = {
+                    let ctx = ctx.clone();
+                    <Self::T as Monad>::bind(ft, move |t| ctx.join_key_tree(tll, kl, t))
+                };
+                let ft = {
+                    let ctx = ctx.clone();
+                    <Self::T as Monad>::bind(ft, move |t| ctx.join_key_tree(t, kr, trr))
+                };
+                ft
+            },
+            self.resolve(&rl),
+            self.resolve(&rr),
+        ))
+    }
+
+    fn split_key_empty(
         &self,
         tree: Self::Tree,
         key: KeyRc<'a, Self>,
     ) -> BTWrap<'a, Self, KeySplit<'a, Self>>;
+
+    fn split_key_node(
+        self,
+        node: Self::Node,
+        key: KeyRc<'a, Self>,
+    ) -> BTWrap<'a, Self, KeySplit<'a, Self>> {
+        let (tl, tr, k) = self.split(&node);
+        match self.comparator().pick_smaller(&key, &k) {
+            Comparison::L => {
+                <Self::T as Monad>::bind(self.clone().split_key(tl, key), move |(tll, tlr)| {
+                    <Self::T as Functor>::fmap(|t| (tll, t), self.join_key_tree(tlr, k, tr))
+                })
+            }
+            Comparison::E => <Self::T as Pure>::pure((tl, tr)),
+            Comparison::R => {
+                <Self::T as Monad>::bind(self.clone().split_key(tr, key), move |(trl, trr)| {
+                    <Self::T as Functor>::fmap(|t| (t, trr), self.join_key_tree(tl, k, trl))
+                })
+            }
+        }
+    }
+
+    fn split_key(
+        self,
+        tree: Self::Tree,
+        key: KeyRc<'a, Self>,
+    ) -> BTWrap<'a, Self, KeySplit<'a, Self>> {
+        match self.refer(&tree) {
+            Some(reference) => <Self::T as Monad>::bind(self.resolve(&reference), |node| {
+                self.split_key_node(node, key)
+            }),
+            None => self.split_key_empty(tree, key),
+        }
+    }
+
+    fn add(self, tree: Self::Tree, key: KeyRc<'a, Self>) -> BTWrap<'a, Self, Self::Node> {
+        <Self::T as Monad>::bind(self.clone().split_key(tree, key.clone()), |(tl, tr)| {
+            self.join_key(tl, key, tr)
+        })
+    }
+
+    fn add_tree(self, tree: Self::Tree, key: KeyRc<'a, Self>) -> BTWrap<'a, Self, Self::Tree> {
+        <Self::T as Monad>::bind(self.clone().add(tree, key), move |n| self.tree_of(n))
+    }
+}
+
+pub trait BinaryTreesAvl<'a>: BinaryTrees<'a> {
+    fn height(&self, tree: &Self::Tree) -> u64;
+
+    fn leaf_height_error<T: 'a>(&self, height: u64) -> BTWrap<'a, Self, T>;
+
+    fn assume_node(&self, tree: &Self::Tree) -> BTWrap<'a, Self, Self::Node> {
+        match self.refer(tree) {
+            Some(reference) => self.resolve(&reference),
+            None => self.leaf_height_error(self.height(tree)),
+        }
+    }
+
+    fn join_key_unbalanced(
+        &self,
+        tl: Self::Tree,
+        key: KeyRc<'a, Self>,
+        tr: Self::Tree,
+    ) -> BTWrap<'a, Self, Self::Node>;
+
+    fn join_key_balanced_tree(
+        self,
+        tl: Self::Tree,
+        key: KeyRc<'a, Self>,
+        tr: Self::Tree,
+    ) -> BTWrap<'a, Self, Self::Tree> {
+        <Self::T as Monad>::bind(self.clone().join_key_balanced(tl, key, tr), move |node| {
+            self.tree_of(node)
+        })
+    }
+
+    fn join_key_balanced(
+        self,
+        tl: Self::Tree,
+        key: KeyRc<'a, Self>,
+        tr: Self::Tree,
+    ) -> BTWrap<'a, Self, Self::Node> {
+        let (lh, rh) = (self.height(&tl), self.height(&tr));
+        match (lh.saturating_sub(rh), rh.saturating_sub(lh)) {
+            (0, 0) | (0, 1) | (1, 0) => self.join_key_unbalanced(tl, key, tr),
+            (0, _) => <Self::T as Monad>::bind(self.assume_node(&tr), move |nr| {
+                let (trl, trr, kr) = self.split(&nr);
+                let (rlh, rrh) = (self.height(&trl), self.height(&trr));
+                if rlh > rrh {
+                    <Self::T as Monad>::bind(self.assume_node(&trl), move |nrl| {
+                        let (trll, trlr, krl) = self.split(&nrl);
+                        Self::T::bind2(
+                            self.clone().join_key_balanced_tree(tl, key, trll),
+                            self.clone().join_key_balanced_tree(trlr, kr, trr),
+                            |ti, to| self.join_key_balanced(ti, krl, to),
+                        )
+                    })
+                } else {
+                    <Self::T as Monad>::bind(
+                        self.clone().join_key_balanced_tree(tl, key, trl),
+                        |t| self.join_key_balanced(t, kr, trr),
+                    )
+                }
+            }),
+            (_, 0) => <Self::T as Monad>::bind(self.assume_node(&tl), move |nl| {
+                let (tll, tlr, kl) = self.split(&nl);
+                let (llh, lrh) = (self.height(&tll), self.height(&tlr));
+                if llh < lrh {
+                    <Self::T as Monad>::bind(self.assume_node(&tlr), move |nlr| {
+                        let (tlrl, tlrr, klr) = self.split(&nlr);
+                        Self::T::bind2(
+                            self.clone().join_key_balanced_tree(tll, kl, tlrl),
+                            self.clone().join_key_balanced_tree(tlrr, key, tr),
+                            |to, ti| self.join_key_balanced(to, klr, ti),
+                        )
+                    })
+                } else {
+                    <Self::T as Monad>::bind(
+                        self.clone().join_key_balanced_tree(tlr, key, tr),
+                        |t| self.join_key_balanced(tll, kl, t),
+                    )
+                }
+            }),
+            (_, _) => unreachable!(),
+        }
+    }
 }
diff --git a/src/flow/binary/avl.rs b/src/flow/binary/avl.rs
new file mode 100644
index 0000000..8e9ea24
--- /dev/null
+++ b/src/flow/binary/avl.rs
@@ -0,0 +1,172 @@
+use std::{fmt::Display, marker::PhantomData, rc::Rc};
+
+use crate::flow::comparator::*;
+
+use super::*;
+
+struct AvlN<A> {
+    l: AvlT<A>,
+    r: AvlT<A>,
+    key: Rc<A>,
+}
+
+impl<A> Clone for AvlN<A> {
+    fn clone(&self) -> Self {
+        Self {
+            l: self.l.clone(),
+            r: self.r.clone(),
+            key: self.key.clone(),
+        }
+    }
+}
+
+impl<A: Display> Display for AvlN<A> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_fmt(format_args!("({} {} {})", self.l, self.key, self.r))
+    }
+}
+
+struct AvlR<A> {
+    node: Rc<AvlN<A>>,
+}
+
+impl<A> Clone for AvlR<A> {
+    fn clone(&self) -> Self {
+        Self {
+            node: self.node.clone(),
+        }
+    }
+}
+
+impl<A: Display> Display for AvlR<A> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_fmt(format_args!("{}", self.node))
+    }
+}
+
+struct AvlT<A> {
+    reference: Option<AvlR<A>>,
+    height: u64,
+}
+
+impl<A> Clone for AvlT<A> {
+    fn clone(&self) -> Self {
+        Self {
+            reference: self.reference.clone(),
+            height: self.height,
+        }
+    }
+}
+
+impl<A: Display> Display for AvlT<A> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match &self.reference {
+            Some(reference) => f.write_fmt(format_args!("{}", reference)),
+            None => f.write_fmt(format_args!("-")),
+        }
+    }
+}
+
+struct AvlTs<A> {
+    _a: PhantomData<A>,
+}
+
+impl<A> AvlTs<A> {
+    fn new() -> Self {
+        Self { _a: PhantomData }
+    }
+}
+
+impl<A> Clone for AvlTs<A> {
+    fn clone(&self) -> Self {
+        Self::new()
+    }
+}
+
+impl<'a, A: 'a + PartialOrd> BinaryTrees<'a> for AvlTs<A> {
+    type Node = AvlN<A>;
+
+    type Reference = AvlR<A>;
+
+    type Tree = AvlT<A>;
+
+    type Key = A;
+
+    type Comparator = DefaultComparator;
+
+    type T = instances::solo::SoloInstance;
+
+    fn comparator(&self) -> &Self::Comparator {
+        &DefaultComparator
+    }
+
+    fn split(&self, node: &Self::Node) -> Split<'a, Self> {
+        (node.l.clone(), node.r.clone(), node.key.clone())
+    }
+
+    fn tree_of(&self, node: Self::Node) -> BTWrap<'a, Self, Self::Tree> {
+        AvlT {
+            height: std::cmp::max(node.l.height, node.r.height)
+                .checked_add(1)
+                .unwrap(),
+            reference: Some(AvlR { node: node.into() }),
+        }
+    }
+
+    fn resolve(&self, reference: &Self::Reference) -> BTWrap<'a, Self, Self::Node> {
+        reference.node.as_ref().clone()
+    }
+
+    fn equal(&self, _rhs: &Self::Reference, _lhs: &Self::Reference) -> bool {
+        false
+    }
+
+    fn refer(&self, tree: &Self::Tree) -> Option<Self::Reference> {
+        tree.reference.clone()
+    }
+}
+
+impl<'a, A: 'a + PartialOrd> BinaryTreesMutable<'a> for AvlTs<A> {
+    fn empty(&self) -> Self::Tree {
+        AvlT {
+            reference: None,
+            height: 0,
+        }
+    }
+
+    fn join_key(
+        self,
+        tl: Self::Tree,
+        key: KeyRc<'a, Self>,
+        tr: Self::Tree,
+    ) -> BTWrap<'a, Self, Self::Node> {
+        self.join_key_balanced(tl, key, tr)
+    }
+
+    fn split_key_empty(
+        &self,
+        _tree: Self::Tree,
+        _key: KeyRc<'a, Self>,
+    ) -> BTWrap<'a, Self, KeySplit<'a, Self>> {
+        (self.empty(), self.empty())
+    }
+}
+
+impl<'a, A: 'a + PartialOrd> BinaryTreesAvl<'a> for AvlTs<A> {
+    fn height(&self, tree: &Self::Tree) -> u64 {
+        tree.height
+    }
+
+    fn leaf_height_error<T: 'a>(&self, height: u64) -> BTWrap<'a, Self, T> {
+        panic!("leaf height error: {height}.")
+    }
+
+    fn join_key_unbalanced(
+        &self,
+        tl: Self::Tree,
+        key: KeyRc<'a, Self>,
+        tr: Self::Tree,
+    ) -> BTWrap<'a, Self, Self::Node> {
+        AvlN { l: tl, r: tr, key }
+    }
+}
diff --git a/src/flow/comparator.rs b/src/flow/comparator.rs
index e398914..493e9f8 100644
--- a/src/flow/comparator.rs
+++ b/src/flow/comparator.rs
@@ -10,7 +10,7 @@ pub enum Comparison {
 /// Returns [`Comparison`] saying which value is smaller.
 ///
 /// ```rust
-/// # use radn_rs::flow::traversible::*;
+/// # use radn_rs::flow::comparator::*;
 /// 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);
diff --git a/src/func.rs b/src/func.rs
index dcda31b..1536899 100644
--- a/src/func.rs
+++ b/src/func.rs
@@ -148,6 +148,14 @@ pub trait MonadExt<'a>: Monad<'a> {
     ) -> Self::F<B> {
         Self::iterate(BindableMut::new(a, f))
     }
+
+    fn bind2<A: 'a, B: 'a, C: 'a>(
+        fa: Self::F<A>,
+        fb: Self::F<B>,
+        f: impl 'a + FnOnce(A, B) -> Self::F<C>,
+    ) -> Self::F<C> {
+        Self::join(Self::la2(f, fa, fb))
+    }
 }
 
 impl<'a, T: Monad<'a>> MonadExt<'a> for T {}
diff --git a/src/rstd.rs b/src/rstd.rs
index 11b433d..9c6a832 100644
--- a/src/rstd.rs
+++ b/src/rstd.rs
@@ -36,23 +36,3 @@ impl<S: Serializable> ExtSerializable for S {
         vec
     }
 }
-
-/// Extention trait for simpler conversion between [`Context::T`] and [`Context::Fallible`].
-///
-/// Until either Rust type system or [`crate::func`] take serious changes,
-/// this is the preferred way to switch between [Wrapped] and [fallible].
-pub trait FallibleContext<'a>: Context<'a> {
-    /// Convert a fallible wrapped into a wrapped result.
-    fn unstuff<A: 'a, E: 'a>(
-        wa: WrapE<'a, A, E, Self::Fallible>,
-    ) -> Wrap<'a, Result<A, E>, Self::T> {
-        Self::Fallible::unstuff(wa)
-    }
-
-    /// Convert a wrapped result into a fallible wrapped.
-    fn stuff<A: 'a, E: 'a>(fa: Wrap<'a, Result<A, E>, Self::T>) -> WrapE<'a, A, E, Self::Fallible> {
-        Self::Fallible::stuff(fa)
-    }
-}
-
-impl<'a, Ctx: Context<'a>> FallibleContext<'a> for Ctx {}
diff --git a/src/rstd/collections/avl/bounds.rs b/src/rstd/collections/avl/bounds.rs
index bfc929d..39528dc 100644
--- a/src/rstd/collections/avl/bounds.rs
+++ b/src/rstd/collections/avl/bounds.rs
@@ -1,4 +1,5 @@
-use crate::{flow::comparator::*, func::*};
+use crate::flow::comparator::*;
+use crate::func::*;
 
 use super::{binary::*, *};
 
@@ -146,6 +147,13 @@ impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx>> BoundNode<'a, Ctx, A> {
         Ok(BoundTree { tree, bounds })
     }
 
+    pub fn into_tree_resolution(self) -> BrTree<'a, Ctx, A> {
+        match self.into_tree() {
+            Ok(tree) => <Ctx::T as Pure>::pure(Ok(tree)),
+            Err(e) => <Ctx::T as Pure>::pure(Err(ResolutionError::Parse(e))),
+        }
+    }
+
     pub fn new(
         l: BoundTree<'a, Ctx, A>,
         r: BoundTree<'a, Ctx, A>,
@@ -161,7 +169,13 @@ impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx>> BoundNode<'a, Ctx, A> {
     }
 }
 
+pub type BoundKeySplit<'a, Ctx, A> = (BoundTree<'a, Ctx, A>, BoundTree<'a, Ctx, A>);
+
 impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx>> BoundTree<'a, Ctx, A> {
+    pub fn height(&self) -> u64 {
+        self.tree.height
+    }
+
     pub fn empty(factory: Fctr<'a, Ctx, A>) -> Self {
         Self {
             tree: AvlTree::empty(factory),
@@ -169,11 +183,44 @@ impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx>> BoundTree<'a, Ctx, A> {
         }
     }
 
+    fn empty_bound(factory: Fctr<'a, Ctx, A>, bounds: Bounds<A>) -> Self {
+        Self {
+            tree: AvlTree::empty(factory),
+            bounds,
+        }
+    }
+
     pub fn reference(&self) -> Option<BoundReference<'a, Ctx, A>> {
         let reference = self.tree.reference()?;
         let bounds = self.bounds.clone();
         Some(BoundReference { reference, bounds })
     }
+
+    fn split_empty(
+        bounds: Bounds<A>,
+        key: &Rc<A>,
+        factory: &Fctr<'a, Ctx, A>,
+        comparator: &impl Comparator<A>,
+    ) -> Result<(Self, Self), BoundsError<A>> {
+        let (bl, br) = bounds.split(key, comparator)?;
+        Ok((
+            Self::empty_bound(factory.clone(), bl),
+            Self::empty_bound(factory.clone(), br),
+        ))
+    }
+
+    pub fn split_empty_res(
+        self,
+        key: Rc<A>,
+        factory: Fctr<'a, Ctx, A>,
+        comparator: Rc<impl 'a + Comparator<A>>,
+    ) -> BrKeySplit<'a, Ctx, A> {
+        <Ctx::T as Pure>::pure(
+            Self::split_empty(self.bounds, &key, &factory, comparator.as_ref())
+                .map_err(BoundError::Bounds)
+                .map_err(ResolutionError::Parse),
+        )
+    }
 }
 
 impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx>> Clone for BoundTree<'a, Ctx, A> {
@@ -188,13 +235,18 @@ impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx>> Clone for BoundTree<'a, Ctx,
 pub type BoundResolutionError<'a, Ctx, A> =
     ResolutionError<<Ctx as Context<'a>>::LookupError, BoundErrorA<'a, Ctx, A>>;
 
-pub type BoundResolutionResult<'a, Ctx, A> =
-    Result<BoundNode<'a, Ctx, A>, BoundResolutionError<'a, Ctx, A>>;
+pub type BoundResolutionResult<'a, Ctx, A, T> = Result<T, BoundResolutionError<'a, Ctx, A>>;
 
-pub type BoundResolution<'a, Ctx, A> = Wrapped<'a, Ctx, BoundResolutionResult<'a, Ctx, A>>;
+pub type BoundResolution<'a, Ctx, A, T> = Wrapped<'a, Ctx, BoundResolutionResult<'a, Ctx, A, T>>;
+
+pub type BrNode<'a, Ctx, A> = BoundResolution<'a, Ctx, A, BoundNode<'a, Ctx, A>>;
+
+pub type BrTree<'a, Ctx, A> = BoundResolution<'a, Ctx, A, BoundTree<'a, Ctx, A>>;
+
+pub type BrKeySplit<'a, Ctx, A> = BoundResolution<'a, Ctx, A, BoundKeySplit<'a, Ctx, A>>;
 
 impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx>> BoundReference<'a, Ctx, A> {
-    pub fn resolve(&self, comparator: Rc<impl 'a + Comparator<A>>) -> BoundResolution<'a, Ctx, A> {
+    pub fn resolve(&self, comparator: Rc<impl 'a + Comparator<A>>) -> BrNode<'a, Ctx, A> {
         let bounds = self.bounds.clone();
         <Ctx::T as Functor>::fmap(
             move |resolution| {
diff --git a/src/rstd/collections/avl/context.rs b/src/rstd/collections/avl/context.rs
new file mode 100644
index 0000000..e1e4a6d
--- /dev/null
+++ b/src/rstd/collections/avl/context.rs
@@ -0,0 +1,111 @@
+use crate::flow::{binary::*, comparator::*};
+use crate::func::*;
+use crate::rstd::fallible::*;
+
+use super::{bounds::*, *};
+
+pub struct BoundContext<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx>, C: 'a + Comparator<A>> {
+    factory: A::Fctr,
+    comparator: Rc<C>,
+}
+
+impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx>, C: 'a + Comparator<A>> Clone
+    for BoundContext<'a, Ctx, A, C>
+{
+    fn clone(&self) -> Self {
+        Self {
+            factory: self.factory.clone(),
+            comparator: self.comparator.clone(),
+        }
+    }
+}
+
+impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx>, C: 'a + Comparator<A>> BinaryTrees<'a>
+    for BoundContext<'a, Ctx, A, C>
+{
+    type Node = BoundNode<'a, Ctx, A>;
+    type Reference = BoundReference<'a, Ctx, A>;
+    type Tree = BoundTree<'a, Ctx, A>;
+    type Key = A;
+    type Comparator = C;
+
+    type T = FallibleMonad<'a, Ctx, BoundResolutionError<'a, Ctx, A>>;
+
+    fn comparator(&self) -> &Self::Comparator {
+        &self.comparator
+    }
+
+    fn split(&self, node: &Self::Node) -> Split<'a, Self> {
+        node.split()
+    }
+
+    fn tree_of(&self, node: Self::Node) -> BTWrap<'a, Self, Self::Tree> {
+        match node.into_tree() {
+            Ok(tree) => <Self::T as Pure>::pure(tree),
+            Err(e) => <Self::T as Fail<_>>::fail(ResolutionError::Parse(e)),
+        }
+    }
+
+    fn resolve(&self, reference: &Self::Reference) -> BTWrap<'a, Self, Self::Node> {
+        Ctx::stuff(reference.resolve(self.comparator.clone()))
+    }
+
+    fn equal(&self, rhs: &Self::Reference, lhs: &Self::Reference) -> bool {
+        BoundReference::equal(lhs, rhs, self.comparator.as_ref())
+    }
+
+    fn refer(&self, tree: &Self::Tree) -> Option<Self::Reference> {
+        tree.reference()
+    }
+}
+
+impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx>, C: 'a + Comparator<A>> BinaryTreesMutable<'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: KeyRc<'a, Self>,
+        tr: Self::Tree,
+    ) -> BTWrap<'a, Self, Self::Node> {
+        self.join_key_balanced(tl, key, tr)
+    }
+
+    fn split_key_empty(
+        &self,
+        tree: Self::Tree,
+        key: KeyRc<'a, Self>,
+    ) -> BTWrap<'a, Self, KeySplit<'a, Self>> {
+        Ctx::stuff(tree.split_empty_res(key, self.factory.clone(), self.comparator.clone()))
+    }
+}
+
+impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx>, C: 'a + Comparator<A>> BinaryTreesAvl<'a>
+    for BoundContext<'a, Ctx, A, C>
+{
+    fn height(&self, tree: &Self::Tree) -> u64 {
+        tree.height()
+    }
+
+    fn leaf_height_error<T: 'a>(&self, height: u64) -> BTWrap<'a, Self, T> {
+        <FallibleMonad<'a, Ctx, _> as Fail<_>>::fail(ResolutionError::Parse(BoundError::Avl(
+            AvlError::LeafHeight(height),
+        )))
+    }
+
+    fn join_key_unbalanced(
+        &self,
+        tl: Self::Tree,
+        key: KeyRc<'a, Self>,
+        tr: Self::Tree,
+    ) -> BTWrap<'a, Self, Self::Node> {
+        match BoundNode::new(tl, tr, key, self.comparator.as_ref()) {
+            Ok(n) => <FallibleMonad<'a, Ctx, _> as Pure>::pure(n),
+            Err(e) => <FallibleMonad<'a, Ctx, _> as Fail<_>>::fail(ResolutionError::Parse(e)),
+        }
+    }
+}
diff --git a/src/rstd/collections/rbtree/subset.rs b/src/rstd/collections/rbtree/subset.rs
index 5ceca36..effcc77 100644
--- a/src/rstd/collections/rbtree/subset.rs
+++ b/src/rstd/collections/rbtree/subset.rs
@@ -2,7 +2,7 @@ use std::rc::Rc;
 
 use crate::flow::traversible::*;
 use crate::func::*;
-use crate::rstd::{fallible::*, *};
+use crate::rstd::fallible::*;
 
 use super::*;
 
diff --git a/src/rstd/fallible.rs b/src/rstd/fallible.rs
index cb32a31..c777fd6 100644
--- a/src/rstd/fallible.rs
+++ b/src/rstd/fallible.rs
@@ -7,3 +7,23 @@ pub type FallibleMonad<'a, Ctx, E> = <<Ctx as Context<'a>>::Fallible as MonadFai
 
 /// Preferred [Wrapped] [Result].
 pub type FallibleWrapped<'a, Ctx, A, E> = Wrap<'a, A, FallibleMonad<'a, Ctx, E>>;
+
+/// Extention trait for simpler conversion between [`Context::T`] and [`Context::Fallible`].
+///
+/// Until either Rust type system or [`crate::func`] take serious changes,
+/// this is the preferred way to switch between [Wrapped] and [fallible].
+pub trait FallibleContext<'a>: Context<'a> {
+    /// Convert a fallible wrapped into a wrapped result.
+    fn unstuff<A: 'a, E: 'a>(
+        wa: WrapE<'a, A, E, Self::Fallible>,
+    ) -> Wrap<'a, Result<A, E>, Self::T> {
+        Self::Fallible::unstuff(wa)
+    }
+
+    /// Convert a wrapped result into a fallible wrapped.
+    fn stuff<A: 'a, E: 'a>(fa: Wrap<'a, Result<A, E>, Self::T>) -> WrapE<'a, A, E, Self::Fallible> {
+        Self::Fallible::stuff(fa)
+    }
+}
+
+impl<'a, Ctx: Context<'a>> FallibleContext<'a> for Ctx {}