title: Meaning of the `from` parameter of `lua_resume`
body: <p>From <a href="http://www.lua.org/manual/5.2/manual.html#lua_resume" rel="noreferrer">Lua 5.2 Reference Manual</a>:</p>
  
  <blockquote>
  <pre><code>int lua_resume (lua_State *L, lua_State *from, int nargs);
  </code></pre>
    
    <p>[...]</p>
    
    <p>The parameter <code>from</code> represents the coroutine that is resuming <code>L</code>. If there is no such coroutine, this parameter can be <code>NULL</code>.</p>
  </blockquote>
  
  <p>But it does not say much to me. What exactly does it do? In what circumstances I <em>must</em> pass anything other than NULL?</p>