!7

sbool isConceptRefValid(Concept src, Concept dest) {
  ret src != null && dest != null && src._concepts != null && src._concepts == dest._concepts;
}

sO conceptRefMutex(Concept c) {
  ret or(c._concepts, dummyMutex());
}

concept Thing {
  Thing ref_ptr;
  
  // setter
  selfType ref(Thing dest) {
    synchronized(conceptRefMutex(this)) {
      if (ref_ptr == dest) ret;
      unindexConceptRef(this, ref_ptr);
      ref_ptr = thing;
      indexConceptRef(this, thing);
    }
    this;
  }
  
  // getter. we could get rid of this by making the field
  // publicly accessible, but then we run the slight chance
  // of setting it without the setter inadvertently.
  Thing ref() { ret ref_ptr; }
}

cprint {}