footnote method
The number footnote label displays as — its position among the
document's footnote definitions, from 1 — or null when it has none.
Implementation
int? footnote(String label) {
final lookups = _lookups;
if (lookups == null && _footnotes.isEmpty) {
return null;
}
final key = normalizeLabel(label);
lookups?.add('f:$key');
return _footnotes[key];
}