(1) A firm or permit holder shall not express an opinion that financial statements are presented in conformity with generally accepted accounting principles if such financial statements contain any departure from such accounting principles which has a material effect on the financial statements taken as a whole, unless the firm or permit holder can demonstrate that by reason of unusual circumstances the financial statements would otherwise have been misleading. In such a case, the firm or permit holder's report must describe the departure, the approximate effects thereof, if practicable, and the reasons why compliance with the principle would result in a misleading statement. For purposes of this rule generally accepted accounting principles are considered to be defined by pronouncements issued by the financial accounting standards board and its predecessor entities and similar pronouncements issued by other entities having similar generally recognized authority.