MANILA, Philippines — Senator-judge Alan Peter Cayetano on Monday said that an impartial tribunal is a prerequisite for holding public officials accountable as the Senate impeachment court continued hearing the prosecution’s case against Vice President Sara Duterte.In his exchange with prosecution witness retired Sandiganbayan Presiding Justice Amparo Cabotaje-Tang, Cayetano said lack of impartiality could put accountability at risk, particularl…