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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 permission_to_perm(permission):
 """ Convert a permission instance to a permission-string. Examples -------- 'auth.add_user' """

    app_label = permission.content_type.app_label
    codename = permission.codename
    return '%s.%s' % (app_label, codename)