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. 2019 May 2;21(5):459. doi: 10.3390/e21050459

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© 2019 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Considering the power set of occurrences. All subsets $x_{t - 1} \subseteq v_{t - 1}$ and $y_{t} \subseteq v_{t}$ are considered as occurrences which may have an actual effect or an actual cause.