= Moments of the mutation count on a neutral coalescent tree
{title2=$\mathbb ES=\theta H_{n-1},\quad\operatorname{Var}S=\theta H_{n-1}+\theta^2H_{n-1}^{(2)}$}
The <independent mutation counts in coalescent epochs> have the <geometric mutation count in a coalescent epoch> laws. Summing their <expectations> and <variances> gives the displayed formulas, where $H_m=\sum_{i=1}^m1/i$ and $H_m^{(2)}=\sum_{i=1}^m1/i^2$. The extra quadratic term records randomness of the ancestral tree, beyond the conditional <Poisson distribution> variance.
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