S. 147: Reassessment-Summary assessment-Assessing Officer not precluded from re-opening assessment-Appeal to Appellate Tribunal Powers of Tribunal challenging finding of Commissioner (Appeals) with regard to jurisdiction of Assessing Officer to reopen assessment-Tribunal possesses ample power to decide any issue going to the root of the subject matter-Assessee’s objection that Tribunal could not have taken upon itself to decide it not tenable [S. 56,,143(1), 148, 254(1), 260A]