android/guava/src/com/google/common/collect/AbstractBiMap.java
void setDelegates(Map<K, V> forward, Map<V, K> backward) {
    checkState(delegate == null);
    checkState(inverse == null);
    checkArgument(forward.isEmpty());
    checkArgument(backward.isEmpty());
    checkArgument(forward != backward);
    delegate = forward;
    inverse = makeInverse(backward);
  }
void setDelegates(Map<K, V> forward, Map<V, K> backward) {
    checkState(delegate == null);
    checkState(inverse == null);
    checkArgument(forward.isEmpty());
    checkArgument(backward.isEmpty());
    checkArgument(forward != backward);
    delegate = forward;
    inverse = makeInverse(backward);
  }
Specifies the delegate maps going in each direction. Called by the constructor and by
subclasses during deserialization.