static <A extends Concept> A nuLike lowestConceptByField(Class<A> c, S field) {
  ret lowestConceptByField(db_mainConcepts(), c, field);
}

static <A extends Concept> A lowestConceptByField(Concepts concepts, Class<A> c, S field) {
  // indexed
  IFieldIndex<A, O> index = concepts.getFieldIndex(c, field);
  if (index cast ConceptFieldIndexDesc)
    ret (A) first(((NavigableMap) index.valueToObject.data).lastEntry());
  ret lowestByField(field, concepts.list(c));
}