Thanks @mikepea, I wasn't sure about the correct notation and couldn't see anything distinguishing the two.

Looking futher, the [Rubydoc documentation](http://ruby-doc.org/documentation-guidelines.html) says:

> Use :: for describing class methods, # for describing instance methods, and use . for example code.

So I've gone with '::', as in `self::validate`. Happy to change this if there's some other convention that we prefer.