anchor.diff:    ! unless the external solver is used.
     integer :: nProcX, nProcY
   
  -  ! integer :: MPI_COMM_WORLD
  +#ifndef useExtSolver
  +  ! if we don't call mpi, then this needs to be expicitly defined
  +  integer :: MPI_COMM_WORLD
  +#endif
     integer :: ierr
     integer :: num_procs, myid
   
  
positive.code: 
  #ifndef useExtSolver
    myid = 0
  #endif
  
  
  
negative.code:       hypre_grid)
    print *, 'Execution ended normally'
    
  #ifdef useExtSolver
    ! Finalize MPI
    call MPI_Finalize(ierr)
  #endif
  
    stop 0
  end program aronnax
  
  