O que é Garman–Kohlhagen model?

The Garman–Kohlhagen model is Black–Scholes for an exchange rate $S$ (units of the quote currency per unit of the base currency): the base currency is an asset paying a continuous yield equal to its interest rate $r_f$, the quote currency’s rate $r_d$ discounts, and a call on the base currency is worth $e^{-r_fT}S\,\Phi(d_1)-e^{-r_dT}K\,\Phi(d_2)$ with $d_{1,2}=\bigl(\ln(F/K)\pm\frac12\sigma^2T\bigr)/(\sigma\sqrt T)$ and $F=Se^{(r_d-r_f)T}$.