Function builtin_dpois 

fn builtin_dpois(
    args: &[RValue],
    named: &[(String, RValue)],
) -> Result<RValue, RError>

Poisson density (probability mass) function.

dpois(x, lambda) = lambda^x * exp(-lambda) / x!

@param x quantile vector (non-negative integers) @param lambda mean rate parameter @return numeric vector of probabilities