code: func (entries Entries) successor(key uint64) (Entry, int) {
  	i := entries.search(key)
  	if i == len(entries) {
  		return nil, -1
  	}
  
  	return entries[i], i
  }
docs: // successor will return the first entry that has a key
  // greater than or equal to provided key.  Also returned
  // is the index of the find.  Returns nil, -1 if a successor does
  // not exist.