static IntRange varyIntRange(IntRange r, int variation, int minLength, int min, int max) {
  // vary start within min/max
  int start = random(max(min, r.start-variation), min(max, r.start+variation)+1);
  // vary end within .../max
  int end = random(r.end-variation, min(max, r.end+variation+1));
  // make longer if too short
  end = max(end, start+minLength);
  // move left if too long
  if (end > max) {
    int shift = end-max;
    start -= shift;
    end -= shift;
  }
  ret intRange(start, end);
}