anchor.diff:      ! 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 
  +    ! 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
  +    !  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.
  
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:     !   
      !   
      !               n              dt     _ 
      !            = q  + 0.5  ( 1 - -- A ) Dq + 0.5 dt H'   
      !               i              dx     
      !     
      !                   +---------+---------+ +---+---+    
  