def __add__(self, value):
<EOL>if type(value) == self.__class__:<EOL><INDENT>new_root = self.__class__()<EOL>new_root.add_child(self)<EOL>new_root.add_child(value)<EOL>return new_root<EOL><DEDENT>else:<EOL><INDENT>raise TreeError("<STR_LIT>")<EOL><DEDENT>
This allows to sum two trees.