if (citation == null) {
addError("Citation is null");
return;
}
if (citation instanceof CitationWithSource) {
CitationWithSource c = (CitationWithSource) citation;
if (c.source == null) {
addError("CitationWithSource requires a non-null source reference", c);
}
checkOptionalString(c.whereInSource, "where within source", c);
checkOptionalString(c.eventCited, "event type cited from", c);