anchor.diff:  
   
         subroutine compute_velocity_psi_comp(mx,my,mbc,
  -     &      dx,dy,t,xlower,ylower,aux,maux)
  +     &      dx,dy,t,blockno,xlower,ylower,aux,maux)
         implicit none
   
  -      integer mx,my,mbc,maux
  +      integer mx,my,mbc,maux,blockno
         double precision dx,dy, t, xlower,ylower
   
         double precision xd1(2),xd2(2)
  
positive.code:             xd2(1) = xlower + (i-1)*dx
              xd2(2) = ylower + (j-1)*dy
  
              call get_vel_psi_comp(xd1,xd2,dy,vn,t)
              aux(i,j,2) = vn
           enddo
        enddo
  
negative.code:             xd2(1) = xlower + (i-1)*dx
              xd2(2) = ylower + (j-1)*dy
  
              call get_vel_psi_comp(blockno,xd1,xd2,dx,vn,t)
              aux(i,j,3) = -vn
           enddo
        enddo
  
        end
  
        subroutine get_vel_psi_comp(blockno,xd1,xd2,ds,vn,t)
        implicit none
  
        double precision xd1(2),xd2(2), ds, vn, psi,t
        integer blockno
  
        vn = (psi(blockno,xd1(1),xd1(2),t) -
       &      psi(blockno,xd2(1),xd2(2),t))/ds
  
        end
  