The aggregate principal amount of bonds and notes issued by the Housing Development Fund shall not exceed $1,250,000,000 outstanding at any one time: Provided, That in computing the total amount of bonds and notes which may at any one time be outstanding, the principal amount of any outstanding bonds or notes refunded or to be refunded either by application of the proceeds of the sale of any refunding bonds or notes of the Housing Development Fund or by exchange for any such refunding bonds or notes, shall be excluded.