nested combination

$H_k^n=\left(\genfrac{}{}{0ex}{}{n+k-1}{k}\right)=\frac{(n+k-1)!}{k!(n-1)!}$

Vertical reversal of duplicate combinations $H_k^n=\left(\genfrac{}{}{0ex}{}{n+k-1}{k}\right)=\left(\genfrac{}{}{0ex}{}{n+k-1}{n-1}\right)=H_{n-1}^k$

combination with repetition combination with replacement comb_rep multi-choose

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