code: func (c *index) SeekFirst(r kv.Retriever) (iter table.IndexIterator, err error) {
  	upperBound := c.prefix.PrefixNext()
  	it, err := r.Iter(c.prefix, upperBound)
  	if err != nil {
  		return nil, err
  	}
  	return &indexIter{it: it, idx: c, prefix: c.prefix}, nil
  }
docs: // SeekFirst returns an iterator which points to the first entry of the KV index.