(protected) r = this >> n*DB
function bnpDRShiftTo(n,r) {
  var this_array = this.array;
  var r_array = r.array;
  for(var i = n; i < this.t; ++i) r_array[i-n] = this_array[i];
  r.t = Math.max(this.t-n,0);
  r.s = this.s;
}