// no null keys or null values
sclass BijectiveMap<A, B> {
  new Map<A, B> forward;
  new Map<B, A> backward;
  
  void put(A a, B b) {
    assertNotNull(a);
    assertNotNull(b);
    A oldKey = backward.get(b);
    if (eq(oldKey, a))
      ret; // nothing to do
    backward.put(b, a);
    forward.put(a, b);
  }
  
  void remove(A a) {
    B b = forward.get(a);
    if (b == null)
      ret; // nothing to do
    backward.remove(b);
    forward.remove(a);
  }
  
  B get(A a) {
    ret forward.get(a);
  }
  
  B inverseGet(B b) {
    ret backward.get(b);
  }
  
  Set<A> keySet() { ret immutableSet(forward.keySet()); }
  Set<B> valueSet() { ret immutableSet(backward.keySet()); }
  
  int size() { ret forward.size(); }
  
  Map<A, B> forwardMap() { ret immutableMap(forward); }
  Map<B, A> backwardMap() { ret immutableMap(backward); }
}