anchor.diff:        double precision xp,yp,pi
         common /compi/ pi
   
  -      u = -2*((sin(pi*xp))**2 * sin(pi*yp) * cos(pi*yp)) - 2*xp
  +C       u = -2*((sin(pi*xp))**2 * sin(pi*yp) * cos(pi*yp)) + 2
  +      u = exp(-(xp-0.5)**2/(2*0.1**2))
   
         return
         end
  
positive.code:       double precision xp,yp,pi
        common /compi/ pi
  
        v = 2*((sin(pi*yp))**2 * sin(pi*xp) * cos(pi*xp)) - 2*yp
  
        return
        end
  
negative.code:          return
        endif
  
  C       vt = cos(pi2*(time+dt/2.d0)/tperiod)
        vt = 1.0
  
        do i = 1-mbc,mx+mbc
           do j = 1-mbc,my+mbc
  