anchor.diff:  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          title="Numerical Solution of the {R}iemann Problem for
   c                 Two-Dimensional Gas Dynamics",
   c          journal="SIAM J. Sci. Comput.",
   c          volume="14",
  
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:        rpv(4) = 1.206045378311055d0
  c
  c      # location of four corners:
         xs = .75d0
         ys = .75d0
  c
         do 15 i=1-mbc,mx+mbc
            xcell = xlower + (i-0.5d0)*dx
  