Stocks stepped back Tuesday, as the nascent uptrend ran into a little trouble. The NYSE composite lost 0.6%, while the S&P 500 shed 0.5%. The Nasdaq fell 0.3%, ending a six-session winning streak. Volume rose from Monday's levels on both main exchanges, handing a distribution day to each main index. The distribution-day tally is still low, with two for the NYSE and one for both the Nasdaq and S&P 500. But it's discouraging to have significant