You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: mark_price.go
    path of file: ./repos/go-binance/v2/futures
    
    the code of the file until where you have to start completion: package futures

import (
	"context"
	"encoding/json"
	"net/http"

	"github.com/adshao/go-binance/v2/common"
)

// PremiumIndexService get premium index
type PremiumIndexService struct {
	c      *Client
	symbol *string
}

// Symbol set symbol
func (s *PremiumIndexService) Symbol(symbol string) *PremiumIndexService {
	s.symbol = &symbol
	return s
}

// Do send request
func (s *PremiumIndexService) Do(ctx context.Context, opts ...RequestOption) (res []*PremiumIndex, err error) {
	r := &requ
    
func (s *PremiumIndexService) Do(ctx context.Context, opts ...RequestOption) (res []*PremiumIndex, err error) {
	r := &request{
		method:   http.MethodGet,
		endpoint: "/fapi/v1/premiumIndex",
		secType:  secTypeNone,
	}
	if s.symbol != nil {
		r.setParam("symbol", *s.symbol)
	}
	data, _, err := s.c.callAPI(ctx, r, opts...)
	data = common.ToJSONList(data)
	if err != nil {
		return []*PremiumIndex{}, err
	}
	res = make([]*PremiumIndex, 0)
	err = json.Unmarshal(data, &res)
	if err != nil {
		return []*PremiumIndex{}, err
	}
	return res, nil
}