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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def clean(self):
        '''
        This code prevents multiple individuals from substituting for the
        same class and class teacher.  It also prevents an individual from
        substituting for a class in which they are a teacher.
        '''
        super(SubstituteReportingForm,self).clean()

        occurrences = self.cleaned_data.get('occurrences',[])
        staffMember = self.cleaned_data.get('staffMember')
        replacementFor = self.cleaned_data.get('replacedStaffMember',[])
        event = self.cleaned_data.get('event')

        for occ in occurrences:
            for this_sub in occ.eventstaffmember_set.all():
                if this_sub.replacedStaffMember == replacementFor:
                    self.add_error('occurrences',ValidationError(_('One or more classes you have selected already has a substitute teacher for that class.'),code='invalid'))

        if event and staffMember:
            if staffMember in [x.staffMember for x in event.eventstaffmember_set.filter(category__in=[getConstant('general__eventStaffCategoryAssistant'),getConstant('general__eventStaffCategoryInstructor')])]:
                self.add_error('event',ValidationError(_('You cannot substitute teach for a class in which you were an instructor.'),code='invalid'))