  The computation of the magnification of a finite source by an arbitrary
gravitational lens can be reduced from a two-dimensional to a one-dimensional
integral using a generalization of Stoke's theorem. For a large source lensed
by a planetary-system whose planet lies at the position where one of the two
images would be in the absence of a planet, the integral can be done
analytically. If the planet lies at the position of the major (unperturbed)
image, the excess flux is the same as it would be for an isolated planet. If
the planet lies at the minor image, there is no excess flux.
