HEAD OF STATE **
(Max on Comcast) The mystery of why Chris Rock can’t make a good movie continues. At least this fitfully amusing political comedy marks an improvement: Rock (who also directed) plays a D.C. alderman chosen to replace a deceased presidential candidate, and the blacker he acts, the more his populist following swells. A lowbrow script battles to a draw with moments of stinging farce, and Bernie Mac swipes every scene he’s in. (PG-13; runs through Nov. 17) TY BURR