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We follow the Vancouver-rules on authorship
(see e.g. http://www.icmje.org/index.html and http://www.codex.vr.se/en/etik2.shtml)
"Authorship credit should be based on
1) substantial contributions to conception and design, or acquisition of data, or analysis and interpretation of data;
2) drafting the article or revising it critically for important intellectual content; and
3) final approval of the version to be published.
Authors should meet conditions 1, 2, and 3." (from the Vancouver-rules on authorship)