eft: -4em; counter-increment: source-line; }
    pre.numberSource code > span > a:first-child::before
      { content: counter(source-line);
        position: relative; left: -1em; text-align: right; vertical-align: baseline;
        border: none; display: inline-block;
        -webkit-touch-callout: none; -webkit-user-select: none;
        -khtml-user-select: none; -moz-user-select: none;
        -ms-user-select: none; user-select: none;
        padding: 0 4px; width: 4em;
        color: #aaaaaa;
      }
 