title: std::ops::Add or core::ops::Add?
body: <p>These two traits (<a href="https://doc.rust-lang.org/std/ops/trait.Add.html" rel="noreferrer">std::ops::Add</a>, <a href="https://doc.rust-lang.org/core/ops/trait.Add.html" rel="noreferrer">core::ops::Add</a>) provide the same functionality, and they both use the same example (both utilize <code>std::ops::Add</code>). Their set of implementors differ somewhat.</p>
  
  <p>Should one default to using <code>std::ops::Add</code>? Why do both, as opposed to one, of them exist?</p>