title: Can explain what is necesariy the extra rule on this PEG
body: <p>I'm playing with <a href="http://pegjs.majda.cz/" rel="nofollow">PEG.js</a> and reading and in the <a href="http://nathansuniversity.com/pegs6.html" rel="nofollow">Nathans University</a> i found a good "explain" how to build my own languaje but i'm stuck with this step</p>
  
  <p>i dont understand the <code>primary</code> can explain me please</p>
  
  <pre><code>start =
      comma
  
  comma = 
      left: additive "," right:comma 
          { return {tag: ",", left:left, right:right}; }
      / additive
  
  additive =
      left:multiplicative "+" right:additive
          { return {tag: "+", left:left, right:right}; }
    / multiplicative
  
  multiplicative =
      left:primary "*" right:multiplicative
          { return {tag: "*", left:left, right:right}; }
    / primary
  
  primary =
      integer
    / "(" comma:comma  ")" # Can explain me why is necesary "(" and ")"
        { return comma; }
  
  integer =
      digits:[0-9]+
          { return parseInt(digits.join(""), 10); }
  </code></pre>
  
  <h2>test</h2>
  
  <pre><code>var parse = wrapExceptions(PEG.buildParser(answer).parse);
  
  assert_eq(parse("1+2"),
      {tag:"+", left:1, right:2},
      "parse 1+2");
  assert_eq(parse("1+2*3"),
      {tag:"+", left:1, right:{tag:"*", left:2, right:3}},
      "parse 1+2*3");
  assert_eq(parse("1,2"),
      {tag:",", left:1, right:2},
      "parse 1,2");
  assert_eq(parse("1,2+3"),
      {tag:",", left:1, right:{tag:"+", left:2, right:3}},
      "parse 1,2+3");
  assert_eq(parse("1*2,3"),
      {tag:",", left:{tag:"*", left:1, right:2}, right:3},
      "parse 1*2,3");
  </code></pre>
  
  <p>my question is why is necesary the <code>/ "(" comma:comma  ")"</code> if the entry does not have parenthesis, if remove that line the last test fails</p>