<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def validate(self, folder):
 """Validate files and folders contained in this folder It validates all of the files and folders contained in this folder if some observers are interested in them. """

        for observer in list(self.observers):
            observer.validate(folder)