Compute the keep score (Equation 5 in the paper).

def get_keep_score(source_counts, prediction_counts, target_counts):
  """Compute the keep score (Equation 5 in the paper)."""
  source_and_prediction_counts = source_counts & prediction_counts
  source_and_target_counts = source_counts & target_counts
  true_positives = sum((source_and_prediction_counts &
                        source_and_target_counts).values())
  selected = sum(source_and_prediction_counts.values())
  relevant = sum(source_and_target_counts.values())
  return _get_fbeta_score(true_positives, selected, relevant)