The fertile window

A model says that ovulation comes $14$ days before the next period, and that the fertile days run from $4$ days before ovulation to $3$ days after it. You will write the model and count the fertile days that the days $10$ to $17$ leave out.

The idea. For a cycle of $28$ days the model gives days $10$ to $17$. For other cycles the window moves.

Your tasks.

1. Finish ovulation_day. It gives the day of ovulation for a cycle of cycle days.
2. Finish fertile_days. It gives the list of the $8$ fertile days.
3. Finish left_out. It gives how many fertile days are outside days $10$ to $17$.