function poisson( rand, lambda ) {
	var p = rand();
	var k = 1;
	while ( p > exp( -lambda ) ) {
		k += 1;
		p *= rand();
	}
	return k - 1;
}
Returns a pseudorandom number drawn from a Poisson distribution.

## Notes

-   Appropriate for \\(lambda < 30\\).

## References

-   Knuth, Donald E. 1997. _The Art of Computer Programming, Volume 2 (3rd Ed.): Seminumerical Algorithms_. Boston, MA, USA: Addison-Wesley Longman Publishing Co., Inc.

@private
@param {PRNG} rand - PRNG for generating uniformly distributed numbers
@param {PositiveNumber} lambda - mean
@returns {NonNegativeInteger} pseudorandom number