code: function bnpMultiplyUpperTo(a,n,r) {
    --n;
    var i = r.t = this.t+a.t-n;
    r.s = 0; // assumes a,this >= 0
    while(--i >= 0) r[i] = 0;
    for(i = Math.max(n-this.t,0); i < a.t; ++i)
      r[this.t+i-n] = this.am(n-i,a[i],r,0,0,this.t+i-n);
    r.clamp();
    r.drShiftTo(1,r);
  }
docs: (protected) r = "this * a" without lower n words, n > 0 "this" should be the larger one if appropriate.