Jets. A MAPLE-package for formal differential geometry. The MAPLE-package jets was first designed to be an extension of the package desolv. In the current stage it became an independent package going beyond symmetries to handle different aspects of formal differential geometry, including some important parts of the variational bicomplex. We demonstrate this by computing the set of all Hamiltonian structures of a order at most 3, which are compatible with D x . This set includes among others the famous KdV- operator D xxx +2 3uD x +1 3u x .