$\mathscr{N}_{yoho}$Let $X$ be a set.
I often use $\lvert X\rvert$ as the cardinarity of $X$.
I sometimes use $\# X$ when many vertical bars appear.
And I use $\mathscr{P}(X)$ as the power set of $X$.
I sometimes use $2^X$. It's useful to ask why we write $2^X$ for the power set.
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