This image was uploaded as part of <a href="https://pl.wikipedia.org/wiki/Wikipedia:Wiki_Lubi_e-podr%C4%99czniki" class="extiw" title="pl:Wikipedia:Wiki Lubi e-podręczniki">Wiki Loves e-textbooks</a> contest in Poland.
Christophe Dang Ngoc Chan (original picture), modification: Mielon
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