code: func (c *IterateChain) FirstValue(qs QuadStore) (quad.Value, error) {
  	if qs != nil {
  		c.qs = qs
  	}
  	if c.qs == nil {
  		return nil, errNoQuadStore
  	}
  	v, err := c.First()
  	if err != nil || v == nil {
  		return nil, err
  	}
  	// TODO: return an error from NameOf once we have it exposed
  	return c.qs.NameOf(v), nil
  }
docs: // FirstValue is an analog of First, but it does lookup of a value in QuadStore.