sbool conceptsSortedByField_verbose;

// tries to use index if possible
// result is sorted if index was used
// result is always an independent collection
static <A extends Concept> Cl<A> nuLike conceptsWithFieldLessThan (Class<A> c, S field, O value) {
  ret conceptsWithFieldLessThan(db_mainConcepts(), c, field, value);
}

static <A extends Concept> Cl<A> conceptsWithFieldLessThan(Concepts concepts, Class<A> c, S field, O value) {
  // indexed
  IFieldIndex<A, O> index = concepts.getFieldIndex(c, field);
  if (index cast ConceptFieldIndexDesc) synchronized(index.mutex()) 
    ret reverseInPlace(valuesList(((SortedMap) index.valueToObject).tailMap()));
    
  // filter manually
  ret filter(concepts.list(c), o -> cmp(getOpt(o, field), value) < 0);
}