code: func (yfast *YFastTrie) Successor(key uint64) Entry {
  	entry := yfast.successor(key)
  	if entry == nil {
  		return nil
  	}
  
  	return entry
  }
docs: // Successor returns an Entry with a key equal to or immediately
  // greater than the provided key.  If such an Entry does not exist
  // this returns nil.