docs: def _calc_F_guess(self, alpha, predictions, probabilities):
          """Calculate an estimate of the F-measure based on the scores"""
          num = np.sum(predictions.T * probabilities, axis=1)
          den = np.sum((1 - alpha) * probabilities + \
                       alpha * predictions.T, axis=1)
          F_guess = num/den
          # Ensure guess is not undefined
          F_guess[den==0] = 0.5
          return F_guess