// 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 conceptsWithFieldLessThanOrEqualTo(Class<A> c, S field, O value) {
  ret conceptsWithFieldLessThanOrEqualTo(db_mainConcepts(), c, field, value);
}

static <A extends Concept> Cl<A> conceptsWithFieldLessThanOrEqualTo(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(concatLists(values(((NavigableMap) index.valueToObject.data).tailMap(value, true))));
  }
    
  // filter manually
  ret filter(concepts.list(c), o -> cmp(getOpt(o, field), value) <= 0);
}