anchor.diff:      !   
       !   
       !               n              dt     _ 
  -    !            = q  + 0.5  ( 1 - -- A ) Dq + 0.5 dt H   
  +    !            = q  + 0.5  ( 1 - -- A ) Dq + 0.5 dt H'   
       !               i              dx     
       !     
       !                   +---------+---------+ +---+---+    
  
positive.code:     !
      ! The eigenvalues are u - c, u, u + c
      !
      ! The Jacobian matrix for the primitive variable formulation of the
      ! Euler equations is 
      !
      !       / u   r   0   \  
      !   A = | 0   u   1/r |  
  
negative.code:     ! consider those waves moving toward the interface.  This gives:
      !
      !  n+1/2      ~        dt                        +           -
      ! q         = q  + 0.5 --  sum { l  . [max(lambda , 0) - A ] Dq r  }
      !  i+1/2,L     L       dx   i     i                              i
      !
      ! since l A = lambda l, we have:
      !
      !  n+1/2      ~        dt                   +                     -
      ! q         = q  + 0.5 --  sum { [max(lambda , 0) - lambda ] (l . Dq) r  }
      !  i+1/2,L     L       dx   i                             i    i       i
      !
      ! See Miller & Colella (2002) for more details.  This expression is 
      ! found in Colella (1990) at the bottom of p. 191.
  