第74页 FLOOR/CEILING APPLICATIONS

前一页有两个式子：
\begin{align}
\alpha \leqslant n < \beta  \Longleftrightarrow  \lceil \alpha \rceil \leqslant n < \lceil \beta \rceil \\
\alpha < n \leqslant \beta  \Longleftrightarrow  \lfloor \alpha \rfloor < n \leqslant \lfloor \beta \rfloor
\end{align}

然后高爷爷（？）说：
By the way, there's a mnemonic for remembering which case uses floors and which uses ceilings: Half-open intervals that include the left endpoint but not the right (such as 0 ≤ θ < 1) are slightly more common than those that include the right endpoint but not the left; and floors are slightly more common than ceilings. So by Murphy's Law, the correct rule is the opposite of what we'd expect -- ceilings for [α..β) and floors for (α..β].