static Concept getConcept(long id) {
  ret mainConcepts.getConcept(id);
}

static <A extends Concept> A getConcept(Class<A> cc, long id) {
  ret getConcept(mainConcepts, cc, id);
}

static <A extends Concept> A getConcept(Concepts concepts, Class<A> cc, long id) {
  Concept c = concepts.getConcept(id);
  if (c == null) null;
  if (!isInstance(cc, c))
    fail("Can't convert concept: " + getClassName(c) + " -> " + getClassName(cc) + " (" + id + ")");
  ret (A) c;
}