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diff --git a/fg21sim/extragalactic/clusters/mergertree.py b/fg21sim/extragalactic/clusters/mergertree.py
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+++ b/fg21sim/extragalactic/clusters/mergertree.py
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+# Copyright (c) 2017 Weitian LI <liweitianux@live.com>
+# MIT license
+
+"""
+Merger tree that represents the merging history of a cluster using
+the binary tree data structure.
+"""
+
+import os
+import pickle
+import logging
+
+
+logger = logging.getLogger(__name__)
+
+
+class MergerTree:
+    """
+    A binary tree that represents the cluster merging history.
+
+    Description
+    -----------
+                   Merged (M0, z0, age0)
+                   ~~~~~~~~~~~~~~~~~~~~~
+                   /                   \
+        Main (M1, z1, age1)    Sub (M2, z2, age2)
+        ~~~~~~~~~~~~~~~~~~~    ~~~~~~~~~~~~~~~~~~
+    * "Merged" is the merged cluster from "Main" and "Sub" at redshift
+      z1 (=z2) or cosmic time age1 (=age2).
+      M0 = M1 + M2, M1 > M2
+    * If "Sub" is missing, then this is an accretion event (not a merger).
+
+    Parameters
+    ----------
+    data : dict
+        Data (e.g., mass, redshift, age) associated with this tree node.
+    main, sub : `~MergerTree`
+        Links to the main and sub (optional) clusters between which the
+        merger happens.
+        The ``sub`` cluster may be missing, which is regarded as an
+        accretion event rather than a merger.
+    merged : `~MergerTree`
+        Reverse link to the merged cluster.  Therefore, it is able to refer
+        to the ``sub`` cluster from the ``main`` cluster, and vice versa.
+    """
+    def __init__(self, data, main=None, sub=None, merged=None):
+        self.data = data
+        self.main = main
+        self.sub = sub
+        self.merged = merged
+
+
+def save_mtree(mtree, outfile, clobber=False):
+    """
+    Pickle the merger tree data and save to file.
+    """
+    if os.path.exists(outfile):
+        if clobber:
+            os.remove(outfile)
+            logger.warning("Removed existing file: {0}".format(outfile))
+        else:
+            raise OSError("Output file already exists: {0}".format(outfile))
+    pickle.dump(mtree, open(outfile, "wb"))
+    logger.info("Saved merger tree to file: {0}".format(outfile))
+
+
+def read_mtree(infile):
+    mtree = pickle.load(open(infile, "wb"))
+    logger.info("Loaded merger tree from file: {0}".format(infile))
+    return mtree
+
+
+def plot_mtree(mtree):
+    """
+    Plot the cluster merger tree.
+    """
+    raise NotImplementedError