title: What's gen:start meaning?
body: <p>In project gproc's file gen_leader.erl，a customized behavior is created. But In the following statement, what is module "gen"? I can't find this module in the "erlang document tools <a href="http://www.erlang.org/erldoc" rel="nofollow">http://www.erlang.org/erldoc</a>"? Could you give me some explanation?</p>
  
  <pre><code>behaviour_info(callbacks) -&gt;
      [{init,1},
       {elected,2},
       {surrendered,3},
       {handle_leader_call,4},
       {handle_leader_cast,3},
       {handle_local_only, 4},
       {from_leader,3},
       {handle_call,3},
       {handle_cast,2},
       {handle_DOWN,3},
       {handle_info,2},
       {terminate,2},
       {code_change,4}];
  behaviour_info(_Other) -&gt;
      undefined.
  
  start_link(Name, [_|_] = CandidateNodes, Workers, 
         Mod, Arg, Options) when is_atom(Name) -&gt;
      gen:start(?MODULE, link, {local,Name}, Mod, %&lt;&lt;++++++ What's the meaning?
            {CandidateNodes, Workers, Arg}, Options).
  </code></pre>