With a spectacular (for it, these days) 8% gain in the month, the INC. Index finally made it back to where it began a year ago. So did the fellow-speculative NASDAQ Industrials. But, as anyone who hasn't just got back from Uranus knows, other averages started setting all-time highs way back in January. At least the INC. Index is back on the right track: there have been months when it was the only average to go down. This time, it actually was the leader in percentage gain. To be fair, that is to some degree explainable by simple arithmetic. Each May, coinciding with a new batch of INC. 100 corporations, the INC. Index gets a whole new cast of characters. In effecting a seamless transition, as they say in the mergers-and-acquisitions trade, the fresh batch is shorn of stocks that do not trade on a daily basis, and of stocks that sell for so low a price that their fluctuations distort the results of the simple formula. (Taking the number of shares that INC. "bought" last March, the percentage change in the dollar value of the index is arithmetically calculated each month.) The resultant banishment of 21 relatively lackluster issues perhaps has prejudiced the INC. Index's ebullience, but only for this month.