title: What is "label" in cayley quads?
body: <p>I already tried, to look in <a href="https://godoc.org/github.com/cayleygraph/cayley/quad" rel="nofollow noreferrer">https://godoc.org/github.com/cayleygraph/cayley/quad</a> but cant find the definition of it and it's uses.</p>
  
  <p>I will appreciate any advice and recommendation, thanks.</p>