title: Are mixin parenthesis optional when nested?
body: <p>Consider the following:</p>
  
  <pre><code>.flashing {
  
      .flashKeyFrames(fade-in;{
          0%, 100% {opacity: 1;}
          50% {opacity: 0;}
          });
  
      .flashing(fade-in linear infinite alternate; 1s)
  
  }
  
  .flashKeyFrames(@name; @arguments) {
      @-moz-keyframes @name { @arguments(); }
      @-webkit-keyframes @name { @arguments(); }
      @keyframes @name { @arguments(); }
  }
  
  .flashing(@arguments, @duration) {
      -webkit-animation: @arguments;
      -moz-animation: @arguments;
      animation: @arguments;
  
      -webkit-animation-duration: @duration;
      -moz-animation-duration: @duration;
      animation-duration: @duration;
  }
  </code></pre>
  
  <p>What are the parenthesis necessarily after <code>@arguments</code>? I thought parenthesis were optional when used as mixins? So I'm assuming <code>@arguments</code> isn't being defined as a mixin, but then what is it?</p>