diff --git a/examples/cluster_tree b/examples/cluster_tree
new file mode 100644
index 0000000..b9f2191
--- /dev/null
+++ b/examples/cluster_tree
@@ -0,0 +1,94 @@
+#!/usr/bin/env python3
+
+"""Clustering Model Based on Decision Tree
+"""
+
+import numpy as np
+import numpy.linalg as LA
+from scipy.stats import entropy
+from scipy.spatial.distance import cdist
+
+import pandas as pd
+
+from treelib import Node, Tree
+
+from sklearn.base import ClusterMixin, BaseEstimator
+from sklearn.cluster import KMeans
+
+
+class TreeCluster(BaseEstimator, ClusterMixin, Tree):
+    """Decision Tree for classification/cluster
+
+    epsilon: the threshold of info gain or other select method
+    selection_method: the selection method
+    features_: the features of the input vars
+    classes_: the classes of output vars
+    """
+
+    def __init__(self, epsilon=0.6, features=None, classes=None, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.epsilon = epsilon
+        self.features_ = features
+        self.classes_ = classes
+
+    def fit(self, X, Y=None, mean=None, level=()):
+        """
+        calc cond_proba, proba, priori_proba, features
+        then call fit_with_proba
+        
+        Arguments:
+            X {2D array|list|dataframe} -- input vars
+        
+        Returns:
+            TreeCluster
+        """
+
+        kmeans = KMeans(n_clusters=2)
+
+        if mean is None:
+            mean = X.mean(axis=0)
+        self.add_node(Node(tag='-'.join(map(str, level)), identifier=level, data={'mean':mean}))
+
+        if len(X)>2:
+            kmeans.fit(X)
+            y = kmeans.predict(X)
+            classes_ = np.unique(y)
+            means_ = kmeans.cluster_centers_
+
+            gain = 1 - kmeans.inertia_ / LA.norm(X - mean, 'fro')**2  
+
+            if gain > self.epsilon:
+                for k, m in zip(classes_, means_):
+                    t = TreeCluster(epsilon=self.epsilon)
+                    t.fit(X[y==k], mean=m, level=level+(k,))
+                    self.paste(level, t)
+
+        if level == ():
+            # get cluster centers from the data of the nodes
+            self.cluster_centers_ = [node.data['mean'] for node in self.all_nodes_itr() if node.is_leaf()]
+            self.classes_ = np.arange(len(self.cluster_centers_))
+
+        return self
+
+    def predict_proba(self, X):
+        distances = np.exp(-cdist(X, self.cluster_centers_))
+        return distances / distances.sum(axis=0)[None,:]
+
+    def predict(self, X):
+        p = self.predict_proba(X)
+        return self.classes_[np.argmax(p, axis=1)]
+
+
+if __name__ == '__main__':
+
+    from sklearn import datasets
+
+    iris = datasets.load_iris()
+    X_train, y_train = iris.data, iris.target
+
+    tc = TreeCluster(epsilon=0.5)
+    tc.fit(X_train)
+    y_ = tc.predict(X_train)
+
+    print(tc)
+    print(tc.cluster_centers_)
diff --git a/examples/huffman_tree.py b/examples/huffman_tree.py
new file mode 100644
index 0000000..783e44c
--- /dev/null
+++ b/examples/huffman_tree.py
@@ -0,0 +1,124 @@
+#!/usr/bin/env python
+
+
+"""Huffman coding
+"""
+
+from toolz import concat
+from treelib import Tree, Node
+
+import numpy as np
+
+
+def _get_symbols(tree):
+    """Get `symbols` from the root of a tree or a node
+
+    tree: Tree or Node
+    """
+    if isinstance(tree, Node):
+        a = tree.data["symbols"]
+    else:
+        a = tree.get_node(tree.root).data["symbols"]
+    if isinstance(a, str):
+        return [a]
+    else:
+        return a
+
+
+def _get_frequency(tree):
+    """Get `frequency` from the root of a tree or a node
+
+    tree: Tree or Node
+    """
+    if isinstance(tree, Node):
+        a = tree.data["frequency"]
+    else:
+        a = tree.get_node(tree.root).data["frequency"]
+    if isinstance(a, str):
+        return [a]
+    else:
+        return a
+
+
+def merge(trees, level=""):
+    """merge the trees to one tree by add a root
+
+    Args:
+        trees (list): list of trees or nodes
+        level (tuple, optional): the prefix for identifier
+
+    Returns:
+        Tree
+    """
+
+    data = list(concat(map(_get_symbols, trees)))
+    freq = sum(map(_get_frequency, trees))
+    t = Tree()
+    root = Node(identifier=level, data={"symbols": data, "frequency": freq, "code": ""})
+    t.add_node(root)
+    t.root = level
+    root.tag = f"root: {{{','.join(root.data['symbols'])}}}/{root.data['frequency']}"
+    for k, tree in enumerate(trees):
+        if isinstance(tree, Node):
+            tree.identifier = f"{k}" + tree.identifier
+            tree.data["code"] = f"{k}" + tree.data["code"]
+            tree.tag = f"{tree.data['code']}: {{{','.join(tree.data['symbols'])}}}/{tree.data['frequency']}"
+            t.add_node(tree, parent=level)
+        else:
+            for n in tree.all_nodes_itr():
+                n.identifier = f"{k}" + n.identifier
+                n.data["code"] = f"{k}" + n.data["code"]
+                n.tag = f"{n.data['code']}: {{{','.join(n.data['symbols'])}}}/{n.data['frequency']}"
+
+            tree._nodes = {n.identifier: n for k, n in tree._nodes.items()}
+            tree.root = f"{k}{tree.root}"
+            tid = tree.identifier
+            for n in tree.all_nodes_itr():
+                if n.is_root():
+                    n.set_successors([f"{k}{nid}" for nid in n._successors[tid]], tid)
+                elif n.is_leaf():
+                    n.set_predecessor(f"{k}{n._predecessor[tid]}", tid)
+                else:
+                    n.set_predecessor(f"{k}{n._predecessor[tid]}", tid)
+                    n.set_successors([f"{k}{nid}" for nid in n._successors[tid]], tid)
+
+            t.paste(level, tree, deep=True)
+    return t
+
+
+def huffman_tree(trees, level="", n_branches=2):
+    """Huffman coding
+
+    Args:
+        trees (list): list of trees or nodes
+        level (tuple, optional): the prefix for identifier
+        set n_branches=2 by default
+
+    Returns:
+        Tree: Huffman tree
+    """
+    assert len(trees) >= 2
+
+    if len(trees) == 2:
+        return merge(trees, level=level)
+    else:
+        ks = np.argsort([_get_frequency(tree) for tree in trees])[:n_branches]
+        t = merge([trees[k] for k in ks], level=level)
+        trees = [t, *(tree for k, tree in enumerate(trees) if k not in ks)]
+        return huffman_tree(trees, level=level)
+
+
+def make_node(s, f):
+    """Make `Node` object
+
+    s: str
+    f: number
+    """
+    return Node(identifier="", data={"symbols": s, "frequency": f, "code": ""})
+
+
+d = {"a": 1, "b": 2, "c": 3, "d": 4, "e": 5}
+nodes = [make_node(s, f) for s, f in d.items()]
+nodes = list(nodes)
+t = huffman_tree(nodes)
+print(t)
diff --git a/examples/random_tree.py b/examples/random_tree.py
new file mode 100644
index 0000000..96827e8
--- /dev/null
+++ b/examples/random_tree.py
@@ -0,0 +1,47 @@
+#!/usr/bin/env python3
+
+"""
+Generate a tree randomly; Test the `apply` method;
+"""
+
+import random
+from treelib import Tree
+
+
+def _random(max_depth=5, min_width=1, max_width=2, offset=()):
+    # generate a tree randomly
+    tree = Tree()
+    tree.create_node(identifier=offset)
+    if max_depth == 0:
+        return tree
+    elif max_depth == 1:
+        nb = random.randint(min_width, max_width)
+        for i in range(nb):
+            identifier = offset + (i,)
+            tree.create_node(identifier=identifier, parent=offset)
+    else:
+        nb = random.randint(min_width, max_width)
+        for i in range(nb):
+            _offset = offset + (i,)
+            max_depth -= 1
+            subtree = _random(max_depth=max_depth, max_width=max_width, offset=_offset)
+            tree.paste(offset, subtree)
+    return tree
+
+
+def _map(func, tree):
+    # tree as a functor
+    tree = tree._clone(with_tree=True)
+    print(tree)
+    for a in tree.all_nodes_itr():
+        key(a)
+    return tree
+
+
+def key(node):
+    node.tag = "-".join(map(str, node.identifier))
+
+
+print(_map(key, _random()))
+
+print(_random().apply(key))
diff --git a/treelib/node.py b/treelib/node.py
index 958c734..067f30b 100644
--- a/treelib/node.py
+++ b/treelib/node.py
@@ -267,7 +267,7 @@ def tag(self):
     @tag.setter
     def tag(self, value):
         """Set the value of `_tag`."""
-        self._tag = value if value is not None else None
+        self._tag = value
 
     def __repr__(self):
         name = self.__class__.__name__
diff --git a/treelib/tree.py b/treelib/tree.py
index 1cc9ac8..aadb62c 100644
--- a/treelib/tree.py
+++ b/treelib/tree.py
@@ -336,6 +336,12 @@ def all_nodes_itr(self):
         """
         return self._nodes.values()
 
+    def iternodes(self):
+        """
+        alias of `all_nodes_itr` but conform to the convention of Python.
+        """
+        return self._nodes.values()
+
     def ancestor(self, nid, level=None):
         """
         For a given id, get ancestor node object at a given level.
@@ -377,7 +383,7 @@ def children(self, nid):
 
     def contains(self, nid):
         """Check if the tree contains node of given id"""
-        return True if nid in self._nodes else False
+        return nid in self._nodes
 
     def create_node(self, tag=None, identifier=None, parent=None, data=None):
         """
@@ -515,6 +521,9 @@ def get_node(self, nid):
             return None
         return self._nodes[nid]
 
+    def get_root(self):
+        return self.get_node(self.root)
+
     def is_branch(self, nid):
         """
         Return the children (ID) list of nid.
@@ -689,10 +698,12 @@ def paste(self, nid, new_tree, deep=False):
         if set_joint:
             raise ValueError("Duplicated nodes %s exists." % list(map(text, set_joint)))
 
-        for cid, node in iteritems(new_tree.nodes):
-            if deep:
-                node = deepcopy(new_tree[node])
-            self._nodes.update({cid: node})
+        if deep:
+            new_nodes = {cid: deepcopy(node) for cid, node in iteritems(new_tree.nodes)}
+        else:
+            new_nodes = new_tree.nodes
+        self._nodes.update(new_nodes)
+        for _, node in iteritems(new_nodes):
             node.clone_pointers(new_tree.identifier, self._identifier)
 
         self.__update_bpointer(new_tree.root, nid)
@@ -1003,9 +1014,8 @@ def subtree(self, nid, identifier=None):
             # define nodes parent/children in this tree
             # all pointers are the same as copied tree, except the root
             st[node_n].clone_pointers(self._identifier, st.identifier)
-            if node_n == nid:
-                # reset root parent for the new tree
-                st[node_n].set_predecessor(None, st.identifier)
+        # reset root parent for the new tree
+        st[nid].set_predecessor(None, st.identifier)
         return st
 
     def update_node(self, nid, **attrs):
@@ -1129,6 +1139,32 @@ def to_graphviz(
 
         f.close()
 
+    def apply(self, key, deep=True):
+        """Morphism of tree
+        Work like the built-in `map`
+
+        Arguments
+            key -- impure function of a node
+            deep -- please keep it true
+        """
+        tree = self._clone(with_tree=True, deep=deep)
+        for a in tree.all_nodes():
+            key(a)
+        return tree
+
+    def apply_data(self, key, deep=True):
+        """morphism of tree, but acts on data of nodes.
+        It calls the method `apply`
+
+        Arguments
+            key -- pure function of node.data
+        """
+
+        def _key(a):
+            a.data = key(a.data)
+
+        return self.apply(_key, deep=deep)
+
     @classmethod
     def from_map(cls, child_parent_dict, id_func=None, data_func=None):
         """