tokens: Acknowledgments Guang-Xing Li thanks Xun Shi for discussions and for comments on the paper, and thanks Manuel Behrendt and Andi Burket for introducing him the problem and for sharing their views. Alexei Kritsuk is acknowledged for a discussion at MIAPP (Munich) where he introduced the Marochnik etal. ( 1983 ) paper, and Masataka Fukugita is acknowledged for a discussion on H I /H 2 transition. Jens Kauffman and Pavel Kroupa are acknowledge for critical comments. Bruce Elmegreen is acknowledged for an in-depth discussion on the multiphase ISM. Finally, the referee must be thanked for a careful reading of the paper and for the pertinent comments. This paper is supported by the Deutsche Forschungsgemeinschaft (DFG) priority programme 1573 ISM-SPP.
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