147.23. From the opening of the bid to its expiry, the issuer, his associates and every holder having an interest of over 20% of the class that is the subject of the bid are prohibited from acquiring securities that increase their interest otherwise than according to the terms of the bid; they are also prohibited from accepting undertakings that would enable them to increase their interest on terms other than those of the bid. However, the issuer may redeem securities pursuant to the exemption provided for in paragraph 1 of section 147.21.