anchor.diff:    subroutine Exchange(y, ierr)
       ! declarations
       implicit none
  +    include "mpif.h"
       real*8,  intent(in)  :: y(nxl,nyl)
       integer, intent(out) :: ierr
       integer :: reqSW, reqSE, reqSS, reqSN, reqRW, reqRE, reqRS, reqRN;
  
positive.code:   ! declarations
    use UserData
    implicit none
    real*8,  intent(in)  :: t, rpar
    integer, intent(in)  :: ipar
    integer, intent(out) :: ierr
  
negative.code: ! Main driver program
  !-----------------------------------------------------------------
  program driver
  
    ! inclusions
    use UserData
    implicit none
    include "mpif.h"
  
    ! Declarations
    ! general problem parameters
    real*8,    parameter :: pi = 3.1415926535897932d0
    real*8,    parameter :: T0 = 0.d0     ! initial time 
  