(protected) r = this >> n
function bnpRShiftTo(n,r) {
  var this_array = this.array;
  var r_array = r.array;
  r.s = this.s;
  var ds = Math.floor(n/BI_DB);
  if(ds >= this.t) { r.t = 0; return; }
  var bs = n%BI_DB;
  var cbs = BI_DB-bs;
  var bm = (1<<bs)-1;
  r_array[0] = this_array[ds]>>bs;
  for(var i = ds+1; i < this.t; ++i) {
    r_array[i-ds-1] |= (this_array[i]&bm)<<cbs;
    r_array[i-ds] = this_array[i]>>bs;
  }
  if(bs > 0) r_array[this.t-ds-1] |= (this.s&bm)<<cbs;
  r.t = this.t-ds;
  r.clamp();
}