  We construct an approximate Green's function for $L_{\gamma}=\nabla \cdot
\gamma(x) \nabla u(x)$, which belongs to a class of Fourier Integral Operators
(FIOs) associated to two canonical relations. This leads to analysis of the
composition of two FIOs, associated to a canonical relation with a zero section
problem. The resulting composition is a sum of two FIOs, each associated to two
intersecting canonical relations.
