《JavaScript語言精粹》這書里有個例子沒看懂啊

var block = function ( ) {
  // Remember the current scope. Make a new scope that
  // includes everything from the current one.
  var oldScope = scope;
  scope = Object.create(scope);
  // Advance past the left curly brace.
  advance('{');
  // Parse using the new scope.
  parse(scope);
  // Advance past the right curly brace and discard the
  // new scope, restoring the old one.
  advance('}');
  scope = oldScope;
};

里面的advance()是什么？pdf里面沒有搜索到定義