you ca n't go back , ca n't kill your own grandfather , ca n't shoot lee harvey oswald as he kneels at a sixth-floor window of the texas school book depository , congealing fried chicken on a paper plate beside him and his mail-order rifle aimed , ca n't stop yourself walking across the intersection of mass ave and prospect street with your briefcase in your hand and your copy of the boston phoenix - which you will never read - under your arm .