sclass OneMap<A, B> extends CompactAbstractMap<A, B> {
  A key;
  B value;
  
  public int size() { ret 1; }
  public Set<Map.Entry<A, B>> entrySet() {
    ret litset(simpleMapEntry(key, value));
  }
  
  public bool containsKey(O o) {
    ret isKey(o);
  }
  
  bool isKey(O o) { ret eq(o, key); }
  
  @Override
  public B get(O o) {
    ret isKey(o) ?  value : null;
  }
}