If \( A \) and \( B \) are independent events and \( P\left((A - B) \cup (B - A)\right) = k - P(A)P(B) \), then \( k = \)
The evaluation of the double integral \( \int_{-a}^{a} \int_{0}^{x} \frac{e^y}{(1+e^y)^2} \, dy \, dx \) is: