A family has two children. Find the probability that both children are girls, given that at least one of the two is a girl who was born in winter. Please assume that (i) gender is binary, i.e., a boy or a girl, (ii) P(a boy) = P(a girl) = 1/2, (iii) genders of the two children are independent, i.e., knowing one's gender gives no information about the other's gender, (iv) four seasons are equally likely, and (v) gender is independent of season.