コンピューテーション式
from
F#
computation expression
Monad？do式?に似た記法
http://bleis-tift.hatenablog.com/entry/how-to-make-computation-expression
https://qiita.com/7shi/items/026c7daa5b0b24d02c0f
https://speakerdeck.com/usabarashi/pythondetie-dao-zhi-xiang-puroguramingu?slide=7