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tokens: We are grateful to the referee, Professor M.A. Dopita, for his constructive comments. LSP and EKG acknowledge support in the framework of Sonderforschungsbereich SFB 881 on ‘The Milky Way System’ (especially subproject A5), which is funded by the German Research Foundation (DFG). LSP thanks for the hospitality of the Astronomisches Rechen-Institut at Heidelberg University where part of this investigation was carried out. This work was partly funded by the subsidy allocated to Kazan Federal University for the state assignment in the sphere of scientific activities (LSP).