docs: private void prepareGammaTheta(Vec gamma_i, Vec eLogTheta_i, Vec expLogTheta_i, Random rand)
      {
          final double lambdaInv = (W * K) / (D * 100.0);
          for (int j = 0; j < gamma_i.length(); j++)
              gamma_i.set(j, sampleExpoDist(lambdaInv, rand.nextDouble()) + eta);
  
          expandPsiMinusPsiSum(gamma_i, gamma_i.sum(), eLogTheta_i);
          for (int j = 0; j < eLogTheta_i.length(); j++)
              expLogTheta_i.set(j, FastMath.exp(eLogTheta_i.get(j)));
      }