MGGHAT is callable as a FORTRAN subroutine, so the user must provide a FORTRAN main program that calls mgghat. (Actually, mgghat can be called from a subroutine, but for simplicity, in this section the calling entity will be referred to as the user's main program.) The main program should be in the file user.f. It can be very simple (as short as two lines: call mgghat and end) or a complex program that advances through time, loops through systems, and/or iterates on nonlinear equations and changes user parameters between calls. The only important point is that any program or routine that changes the value of any of the user parameters must contain the statement include `commons'. All the parameters are passed through common blocks in the file commons, so there is no parameter list attached to the call mgghat statement.