code: func (xft *XFastTrie) Predecessor(key uint64) Entry {
  	n := xft.predecessor(key)
  	if n == nil {
  		return nil
  	}
  
  	return n.entry
  }
docs: // Predecessor will return an Entry which matches the provided
  // key or its immediate predecessor.  Will return nil if a predecessor
  // does not exist.  This is an O(log log M) operation.