code: function bnpMultiplyLowerTo(a,n,r) {
    var i = Math.min(this.t+a.t,n);
    r.s = 0; // assumes a,this >= 0
    r.t = i;
    while(i > 0) r[--i] = 0;
    var j;
    for(j = r.t-this.t; i < j; ++i) r[i+this.t] = this.am(0,a[i],r,i,0,this.t);
    for(j = Math.min(a.t,n); i < j; ++i) this.am(0,a[i],r,i,0,n-i);
    r.clamp();
  }
docs: (protected) r = lower n words of "this * a", a.t <= n "this" should be the larger one if appropriate.