static float[] concatFloatArrays(L<float[]> arrays) {
  int l = 0;
  for (float[] a : unnull(arrays)) l += l(a);
  float[] x = new float[l];
  int i = 0;
  for (float[] a : unnull(arrays)) if (a != null) {
    System.arraycopy(a, 0, x, i, l(a));
    i += l(a);
  }
  ret x;
}