* The exponential power distribution method could be speeded up by
using a rational function approximation for the rejection scaling
parameter.

* Do something about the possibility of the user providing invalid
parameters (e.g. negative variance etc). Not sure what to do though,
since returning an error code is not possible. Maybe just return zero.

* Add the triangular distribution.

* Look at Marsaglia & Tsang, "The Monte Python Method for generating
random variables", ACM TOMS Vol 24, No 3, p341
