An ailing Paul Simon stepped out of his retirement from show business Monday night to serenade delegates of the Democratic National Convention in Philadelphia with “A Bridge Over Troubled Water,” at once a plea for unity and a reminder of his own inability to get along with his one-time singing partner Art Garfunkel. The song was the biggest hit for Simon and Garfunkel, but it was virtually the only song Simon’s curly-haired partner sang as lead. Over the years, the song has become like a child being tugged at in a custody fight. Garfunkel sings it to remind Simon he sang their biggest hit and Simon gives interviews in which he goes out of his way to compliment other singers’ versions. A song that was supposed to symbolized unity and our obligations to take care of each other has come to mean something else–and when a sratchy Simon struggled through his rendition, it did not have the effect the convention planners were hoping for. But, that was not all Simon‘s fault, considering what he walked into. Simon was late coming to the stage, and Sen. Al Franken (D.-Minn.) and comedienne Sarah Silverman were asked to stretch out their act, which was not