code: function (pool) {
      var pools = {
        lower: 'abcdefghijklmnopqrstuvwxyz',
        upper: 'ABCDEFGHIJKLMNOPQRSTUVWXYZ',
        number: '0123456789',
        symbol: '!@#$%^&*()[]'
      }
      pools.alpha = pools.lower + pools.upper
      pools['undefined'] = pools.lower + pools.upper + pools.number + pools.symbol
  
      pool = pools[('' + pool).toLowerCase()] || pool
      return pool.charAt(this.natural(0, pool.length - 1))
    }
docs: 返回一个随机字符。