@Override
  public Set<V> values() {
    /*
     * We can almost reuse the inverse's keySet, except we have to fix the
     * iteration order so that it is consistent with the forward map.
     */
    Set<V> result = valueSet;
    return (result == null) ? valueSet = new ValueSet() : result;
  }

  @WeakOuter
  private class ValueSet extends ForwardingSet<V> {

      K conflict = inverse.delegate.putIfAbsent(v, k);
      if (conflict != null) {
        broken = entry;
        // We're definitely going to throw, but we'll try to keep the BiMap in an internally
        // consistent state by removing the bad entry.
        itr.remove();
      }
    }
    if (broken != null) {
      throw new IllegalArgumentException("value already present: " + broken.getValue());
    }
  }