      \fitem{rnd_gamma}($\alpha$:float): returns a random generator that
      produces floating-point values according to a \emph{gamma
        distribution} $P$:
      $$
      P(x | \alpha) = \frac{1}{\Gamma(\alpha)} \, x^{\alpha-1}, 
      \quad x \geq 0.
      $$



      %%% -------------------------------------------------------

