function IDF.GAMMA (P >= 0 && P <= 1, a > 0, b > 0)
= gsl_cdf_gamma_Pinv (P, a, 1. / b);
function PDF.GAMMA (x >= 0, a > 0, b > 0) = gsl_ran_gamma_pdf (x, a, 1. / b);
function IDF.GAMMA (P >= 0 && P <= 1, a > 0, b > 0)
= gsl_cdf_gamma_Pinv (P, a, 1. / b);
function PDF.GAMMA (x >= 0, a > 0, b > 0) = gsl_ran_gamma_pdf (x, a, 1. / b);