anchor.diff:         rpv(4) = 1.206045378311055d0
   c
   c      # location of four corners:
  -       xs = .8d0
  -       ys = .8d0
  +       xs = .75d0
  +       ys = .75d0
   c
          do 15 i=1-mbc,mx+mbc
             xcell = xlower + (i-0.5d0)*dx
  
positive.code:              q(i,j,1) = rpr(iq)
               q(i,j,2) = rpr(iq)*rpu(iq)
               q(i,j,3) = rpr(iq)*rpv(iq)
               q(i,j,4) = rpp(iq)/gamma1 + 0.5d0*rpr(iq)*(rpu(iq)**2 + 
       &                                  rpv(iq)**2)
     15        continue
         return
  
negative.code: c      # 2D Riemann problem from Figure 4 of
  c        @article{csr-col-glaz,
  c          author="C. W. Schulz-Rinne and J. P. Collins and H. M. Glaz",
  c          title="Numerical Solution of the {R}iemann Problem for
  c                 Two-Dimensional Gas Dynamics",
  c          journal="SIAM J. Sci. Comput.",
  c          volume="14",
  