def union(self, label_a, label_b):
<EOL>if label_a == label_b:<EOL><INDENT>return<EOL><DEDENT>root_a = self.find(label_a)<EOL>root_b = self.find(label_b)<EOL>if root_a == root_b:<EOL><INDENT>return<EOL><DEDENT>self.__internal_union(root_a, root_b)<EOL>self.__set_counter -= <NUM_LIT:1><EOL>
Joins two sets into a single new set.
        label_a, label_b can be any nodes within the sets