code: function mult_(x, y) {
    var i;
    if (ss.length!=2*x.length)
      ss=new Array(2*x.length);
    copyInt_(ss, 0);
    for (i=0; i<y.length; i++)
      if (y[i])
        linCombShift_(ss, x, y[i], i);   //ss=1*ss+y[i]*(x<<(i*bpe))
    copy_(x, ss);
  }
docs: do x=x*y for bigInts x and y.  This is faster when y<x.