(Float, Integer, Rational)#round(half: :down)
I learned that The Japanese Ministry of Health, Labor and Welfare requires that the insured burden amount of employment insurance be calculated in round half down mode. Details: http://www.mhlw.go.jp/topics/2005/04/tp0425-2.html
Given there is a real-world situation where half-down is actually mandatory, I think we should implement the (currently lacking)
half: :down option.