Okay, I read that and now I get it. Each live cell has at least three ancestors. Two or fewer of these ancestors come from a forbidden direction, so at least on

Okay, I read that and now I get it. Each live cell has at least three ancestors. Two or fewer of these ancestors come from a forbidden direction, so at least one comes from an allowed direction. Among all of the ancestors from an allowed direction, arbitrarily choose one to be the “mother”. All of the other ancestors are called “fathers”. There's no requirement that a father has to come from a forbidden direction.