@rnd_exponential(λ:float)
P(x | λ) = λ e^(-λ x), x > 0.
See @rnd_bernoulli for a description of random generators.
See also Random Generators @rand @rand_int @random @rnd_bernoulli @rnd_binomial @rnd_exponential @rnd_gamma @rnd_geometric @rnd_normal @rnd_uniform_float @rnd_uniform_int