The story (with my translation) is: François encounter a female berus with just dead prey. The adder eat the mouse (gloups) and walk away a little afraid by the french giant.
This french giant go away for 20 minutes and come back. He is anxious with snakes and didn't recognize the first adder he meet and estimate the same snake with the double of volume.... dont worry, Vipera berus can't have double volume with 20 minutes.... just problem of perception
