}
my $source_id = $source->subfield('901', 'c');
+$source_id = $source->subfield('903', 'a') unless $source_id;
my $target_id = $target->subfield('901', 'c');
+$target_id = $target->subfield('903', 'a') unless $target_id;
my %existing_fields;
foreach my $tag (@$tags) {