title: sed - what is this curly brace notation called?
body: <p>I just found this:</p>
  
  <pre><code>sed '/label/{n;n;s/{}/{some comment}/;}'
  </code></pre>
  
  <p>The intended effect is to seek <code>label</code>, proceed 2 lines down (<code>n;n;</code>) then substitute in (<code>s</code>) <code>some comment</code>.</p>
  
  <p>This is an amazing capability I never knew <code>sed</code> had.</p>
  
  <p>Would someone be kind enough to specify the name of this curly brace notation, and the name of the class of operators inside the braces?</p>