code: private void expandPsiMinusPsiSum(Vec input, double sum, Vec output)
      {
          double psiSum = FastMath.digamma(sum);
          for(int i = 0; i < input.length(); i++)
              output.set(i, FastMath.digamma(input.get(i))-psiSum);
      }
docs: From the 2013 paper, see expectations in figure 5 on page 1323, and
  equation (27) on page 1325
  See also equation 6 in the 2010 paper. 2013 paper figure 5 seems to be a
  typo
  @param input the vector to take the input values from
  @param sum the sum of the {@code input} vector
  @param output the vector to store the transformed inputs in