This would be a breaking change either way, but `binomial(n, k)` with `n < 0` should either produce `(-1).pow(k) * binomial(k - n - 1, k) = n(n-1)...(n-k +1)/k!` or `n` and `k` should be unsigned. See eg https://en.wikipedia.org/wiki/Binomial_coefficient#Generalization_and_connection_to_the_binomial_series.
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