'Enforce expertise as a subset of interests'

def clean_expertise(self):

    interests = set(parse_tags(self.cleaned_data.get('interests', '')))
    expertise = set(parse_tags(self.cleaned_data['expertise']))
    if ((len(expertise) > 0) and (not expertise.issubset(interests))):
        raise forms.ValidationError(_('Areas    of    expertise    must    be    a    subset    of    interests'))
    return self.cleaned_data['expertise']
