General anti-avoidance rule treats arrangements primarily aimed at obtaining tax benefit as impermissible under commercial substance tests. An impermissible avoidance arrangement is one whose main purpose is to obtain a tax benefit and which (a) creates rights or obligations not ordinarily created between persons dealing at arm's length; (b) results in misuse or abuse of the Act's provisions; (c) lacks commercial substance or is deemed to lack it under section 97; or (d) is entered into or carried out by means not ordinarily employed for bona fide purposes. A presumption requires the assessee to disprove that any step primarily aimed to obtain a tax benefit.