This function computes the probability density
of the Truncated Exponential distribution given parameters ($\xi$ and $\alpha$) of the
distribution computed by partexp
. The probability density
function of the distribution is$$f(x) = \frac{\alpha^{-1}\mathrm{exp}(-t/\alpha)}{1 - \mathrm{exp}(-\xi/\alpha)}\mbox{,}$$
where $f(x)$ is the probability density for the quantile $x$,
$\xi$ is a location parameter, $\alpha$ is a scale parameter, and $0 \le x \le \xi$. The distribution has $0 < \tau_2 <= 1="" 2$,="" $\xi=""> 0$, and $1/\alpha \ne 0$.=>