(protected) r = this * a, r != this,a (HAC 14.12) "this" should be the larger one if appropriate.
function bnpMultiplyTo(a,r) {
  var this_array = this.array;
  var r_array = r.array;
  var x = this.abs(), y = a.abs();
  var y_array = y.array;

  var i = x.t;
  r.t = i+y.t;
  while(--i >= 0) r_array[i] = 0;
  for(i = 0; i < y.t; ++i) r_array[i+x.t] = x.am(0,y_array[i],r,i,0,x.t);
  r.s = 0;
  r.clamp();
  if(this.s != a.s) BigInteger.ZERO.subTo(r,r);
}