code: function bnpToRadix(b) {
      if (b == null) b = 10;
      if (this.signum() === 0 || b < 2 || b > 36) return "0";
      var cs = this.chunkSize(b);
      var a = Math.pow(b, cs);
      var d = nbv(a), y = nbi(), z = nbi(), r = "";
      this.divRemTo(d, y, z);
      while (y.signum() > 0) {
          r = (a + z.intValue()).toString(b).substr(1) + r;
          y.divRemTo(d, y, z);
      }
      return z.intValue().toString(b) + r;
  }
docs: (protected) convert to radix string