Compute the deletion score (Equation 6 in the paper).

def get_deletion_score(source_counts, prediction_counts, target_counts, beta=0):
  """Compute the deletion score (Equation 6 in the paper)."""
  source_not_prediction_counts = source_counts - prediction_counts
  source_not_target_counts = source_counts - target_counts
  true_positives = sum((source_not_prediction_counts &
                        source_not_target_counts).values())
  selected = sum(source_not_prediction_counts.values())
  relevant = sum(source_not_target_counts.values())
  return _get_fbeta_score(true_positives, selected, relevant, beta=beta)