docs: public static String[] concat(String[] ...aa) {
      int l = 0;
      for (String[] a : aa) l += a.length;
      String[] r = new String[l];
      l = 0;
      for (String[] a : aa) {
        System.arraycopy(a, 0, r, l, a.length);
        l += a.length;
      }
      return r;
    }