static float[] subFloatArray(float[] b, int start) {
  ret subFloatArray(b, start, l(b));
}
  
static float[] subFloatArray(float[] b, int start, int end) {
  start = max(start, 0); end = min(end, l(b));
  if (start == 0 && end == l(b)) ret b;
  if (start >= end) ret new float[0];
  float[] x = new float[end-start];
  System.arraycopy(b, start, x, 0, end-start);
  ret x;
}