android/guava/src/com/google/common/collect/ForwardingNavigableMap.java
protected Entry<K, V> standardLowerEntry(K key) {
    return headMap(key, false).lastEntry();
  }
protected Entry<K, V> standardLowerEntry(K key) {
    return headMap(key, false).lastEntry();
  }
A sensible definition of {@link #lowerEntry} in terms of the {@code lastEntry()} of {@link
#headMap(Object, boolean)}. If you override {@code headMap}, you may wish to override {@code
lowerEntry} to forward to this implementation.