// We look now at the main file for the tracer problem. We setup two problems in this file and solve them sequentially, first the 1p problem and afterwards the tracer problem. The result of the 1p problem is the pressure distribution in the problem domain. We use it to calculate the volume fluxes, which act as an input for the tracer problem. Based on this volume fluxes, we calculate the transport of a tracer in the following tracer problem.