title: variable _ in lua has a special meaning?
body: <p>I am a newbie to lua / torch. I notice that the variable _ is used a lot, especially in iterators. Example:</p>
  
  <pre><code>for _, node in ipairs(protos.rnn.forwardnodes) do
      a, b = whatever(a,b)
  end
  </code></pre>
  
  <p>this 'variable naming convention' (so to speak) is used in other circumstances as well, as in:</p>
  
  <pre><code>local _,loss = optimizer(feval,params, optim_state)
  </code></pre>
  
  <p>Does _ have any special meaning or is it just one more variable name, among the many possible names?</p>