def __init__(self, items, rules=None):
Cluster.__init__(self, items)<EOL>if rules is None:<EOL><INDENT>self.rules = []<EOL><DEDENT>else:<EOL><INDENT>self.rules = rules<EOL><DEDENT>
Argument:
 | ``items``  --  the items that belong in this cluster

Optional argument:
 | ``rules``  --  a list of rules that binds the items