svoid indexConceptFieldDesc(Class<? extends Concept> c, S field) {
  indexConceptFieldDesc(db_mainConcepts(), c, field);
}

// if there is a non-ordered index, it keeps that one
svoid indexConceptFieldDesc(Concepts concepts, Class<? extends Concept> c, S field) {
  if (!isConceptFieldIndexed(concepts, c, field))
    new ConceptFieldIndexDesc(concepts, c, field);
}