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\newblock {\em Stellar Atmospheres}.
\newblock Freeman, San Francisco, CS, 2nd edition, 1978.

\bibitem{sc85}
G.B. Scharmer and M.~Carlsson.
\newblock A new approach to multi-level non-lte radiative transfer problems.
\newblock {\em Journal of Computational Physics}, {\bf 59}:56--80, 1985.

\end{thebibliography}