code: def build_prev_op(self):
          
          code = self.code
          codelen = len(code)
          # 2.x uses prev 3.x uses prev_op. Sigh
          # Until we get this sorted out.
          self.prev = self.prev_op = [0]
          for offset in self.op_range(0, codelen):
              op = code[offset]
              for _ in range(instruction_size(op, self.opc)):
                  self.prev_op.append(offset)
docs: Compose 'list-map' which allows to jump to previous
          op, given offset of current op as index.