anchor.diff:  
   
         end
  +
  +      subroutine build_transform(transform_ptr,a)
  +      implicit none
  +
  +      integer a(2,2)
  +      integer*8 transform_ptr
  +      integer f(2)
  +      integer mi(4),mj(4)
  +      integer i1,j1
  +
  +c     # Assume index mapping fclaw2d_transform_face_half has the
  +c     # the form
  +c     #
  +c     #       T(ic,jc) = A*(ic,jc) + F = (iff,jff)
  +c     #
  +c     # where (ic,jc) is the coarse grid index, and (iff,jff)
  +c     # is the first fine grid index.
  +c     #
  +c     # We can recover coefficients A(2,2) with the following
  +c     # calls to T.
  +
  +      i1 = 0
  +      j1 = 0
  +      call fclaw2d_transform_face_half(i1,j1,mi,mj,
  +     &      transform_ptr)
  +      f(1) = mi(1)
  +      f(2) = mj(1)
  +
  +      i1 = 1
  +      j1 = 0
  +      call fclaw2d_transform_face_half(i1,j1,mi,mj,
  +     &      transform_ptr)
  +      a(1,1) = mi(1) - f(1)
  +      a(2,1) = mj(1) - f(2)
  +
  +      i1 = 0
  +      j1 = 1
  +      call fclaw2d_transform_face_half(i1,j1,mi,mj,
  +     &      transform_ptr)
  +      a(1,2) = mi(1) - f(1)
  +      a(2,2) = mj(1) - f(2)
  +
  +      end
  
positive.code:       integer i2(0:rr2-1),j2(0:rr2-1)
        logical is_valid_interp
        logical skip_this_grid
  
        mth = 5
        r2 = refratio*refratio
  
negative.code:       double precision qcoarse(1-mbc:mx+mbc,1-mbc:my+mbc,meqn)
  
        integer mq,r2, m
        integer i, ic1, ic2, ibc, ifine,i1
        integer j, jc1, jc2, jbc, jfine,j1
        integer ic_add, jc_add, ic, jc, mth
        double precision gradx, grady, qc, sl, sr, value
        double precision compute_slopes
  
  c     # This should be refratio*refratio.
  