"If at the time of the 'butterfly' reorganization the shareholders had formed the intention to sell their shares, and their shares are eventually sold, the reorganization and the ultimate sale will be considered to form part of the same series even though at the time of the reorganization the shareholders either had not determined all of the important elements of the subsequent sale - such as, for example, the identity of the purchaser or the purchase price and terms of payment - or lacked the ability to implement the subsequent sale."