// f : Concept -> Concept
static Concept conceptGraphMap(Concept c, O f) {
  ret conceptGraphMap_impl(c, f, new IdentityHashMap);
}

static Concept conceptGraphMap_impl(Concept c, O f, IdentityHashMap<Concept, Concept> seen) {
  if (seen.containsKey(c))
    ret seen.get(c);
  Concept d = cast callF(f, c);
  if (d != c) {
    seen.put(c, d);
    ret d;
  }

  d = cnew(c.getClass());
  seen.put(c, d);
  
  for (S field : conceptFields(c)) {
    O val = cget(c, field);
    if (!val instanceof Concept)
      cset(d, field, val);
    else
      cset(d, field, conceptGraphMap_impl(val/Concept, f, seen));
  }
  ret d;
}