Scala: Variances
https://docs.scala-lang.org/tour/variances.html
https://qiita.com/harry0000/items/113b5ddbe64a2835a6e2
(Japanese)
https://qiita.com/f81@github/items/7a664df8f4b87d86e5d8
(Japanese)
https://qiita.com/mtoyoshi/items/bd0ad545935225419327
(Japanese)
#Scala