<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def check_labels(self):
 """ Checks if all the labels has been declared """

        for entry in self.labels:
            self.check_is_declared(entry.name, entry.lineno, CLASS.label)