def add_sister(self, sister=None, name=None, dist=None):
if self.up == None:<EOL><INDENT>raise TreeError("<STR_LIT>")<EOL><DEDENT>else:<EOL><INDENT>return self.up.add_child(child=sister, name=name, dist=dist)<EOL><DEDENT>
Adds a sister to this node. If sister node is not supplied
as an argument, a new TreeNode instance will be created and
returned.