code: function carry_(x) {
    var i, k, c, b;
    k=x.length;
    c=0;
    for (i=0; i<k; i++) {
      c+=x[i];
      b=0;
      if (c<0) {
        b=-(c>>bpe);
        c+=b*radix;
      }
      x[i]=c & mask;
      c=(c>>bpe)-b;
    }
  }
docs: do carries and borrows so each element of the bigInt x fits in bpe bits.