Securities actually issued or assumed by the utility which have been reacquired shall be either retired or carried in account 152, Reacquired Capital Stock, or account 153, Reacquired Long-term Debt, unless it is required by provision of a mortgage, or by decision of a trustee not subject to control by the accounting utility, that they be retained alive in sinking or other funds. When so retained they shall be considered as actually outstanding, but not otherwise.