@rnd_exponential(λ:float)
returns a random generator that produces floats x according to an exponential distribution P of parameter λ:

    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