docs: function parseAst(filename, ast) {
    var directives = [];
    esprimaHelper.traverse(ast, function (node) {
      var calleeExpressionName = esprimaHelper.getCallExpressionName(node);
      if (calleeExpressionName === 'directive') {
        var directiveArguments = node['arguments'] || [];
        if (directiveArguments.length >= 2 && directiveArguments[0].type === 'Literal') {
          directives.push(new Directive(filename, directiveArguments));
        }
      }
    });
    return directives;
  }