### Determining the structure of the system matrices

In mastructf.c the structure of the matrices of the linear equation systems is determined. Indeed, the structure is usually sparse and therefore it is important to know which elements are nonzero. Only these elements are stored. This is the equivalent routine to mastruct.c for solid mechanics applications. However, contrary to solid mechanics the single point constraints and multiple point constraints are not taken into account while calculating the structure of the matrix, i.e. boundary conditions do not reduce the system of equations. So the equations are built and solved in the assumption that no SPC's or MPC's are applied. They are taken into account at a later stage of the calculation. This, however, does not apply to the matrix of the pressure equations for incompressible fluids. In the latter equations the SPC's are taken into account, but not the MPC's. The reason for this is that the pressure equation system is the only system for which a regular linear equation solver such as SPOOLES is used. All other systems are diagonalized (lumped). In the absence of SPC's the solution to the pressure equations is not unique and the corresponding matrix is singular. This cannot be handled by a standard solver.