docs: public static StringBuilder formatTo(StringBuilder buf, double[] d, String sep) {
      if(d == null) {
        return buf.append("null");
      }
      if(d.length == 0) {
        return buf;
      }
      buf.append(d[0]);
      for(int i = 1; i < d.length; i++) {
        buf.append(sep).append(d[i]);
      }
      return buf;
    }