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3. Adler, A., Demaine, E.D., Hesterberg, A., Liu, Q., Rudoy, M.: Clickomania is hard, even with two colors and columns. Math. Various Entertain. Subj. 2, 325–363 (2017)
4. Gualà, L., Leucci, S., Natale, E.: Bejeweled, candy crush and other match-three games are (NP-)hard. In: 2014 IEEE Conference on Computational Intelligence and Games, pp. 1–8 (2014). https://doi.org/10.1109/CIG.2014.6932866
5. Friedman, E.: Pushing blocks in gravity is NP-hard. http://www.stetson.edu/~efriedma/papers/gravity.pdf