// Figure out which mutation to apply.
	whichMut := false
	if req.PredicateFilter == nil {
		// Use true_mutations iff row contains any cells.
		whichMut = !r.isEmpty()
	} else {
		// Use true_mutations iff any cells in the row match the filter.
		// TODO(dsymonds): This could be cheaper.
		nr := r.copy()
		filterRow(req.PredicateFilter, nr)
		whichMut = !nr.isEmpty()
	}

// filterRow modifies a row with the given filter. Returns true if at least one cell from the row matches,
// false otherwise. If a filter is invalid, filterRow returns false and an error.
func filterRow(f *btpb.RowFilter, r *row) (bool, error) {
	if f == nil {
		return true, nil
	}
	// Handle filters that apply beyond just including/excluding cells.
	switch f := f.Filter.(type) {
	case *btpb.RowFilter_BlockAllFilter:
		return !f.BlockAllFilter, nil
	case *btpb.RowFilter_PassAllFilter:
		return f.PassAllFilter, nil