code: func (t *Tree) First() (k []byte, v []byte) {
  	if q := t.first; q != nil {
  		q := &q.d[0]
  		k, v = q.k, q.v
  	}
  	return
  }
docs: // First returns the first item of the tree in the key collating order, or
  // (zero-value, zero-value) if the tree is empty.