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: staking_service.go
    path of file: ./repos/go-binance/v2
    
    the code of the file until where you have to start completion: package binance

import (
	"context"
	"net/http"
)

// StakingProductPositionService fetches the staking product positions
type StakingProductPositionService struct {
	c         *Client
	product   StakingProduct
	productId *string
	asset     *string
	current   *int32
	size      *int32
}

// Product sets the product parameter.
func (s *StakingProductPositionService) Product(product StakingProduct) *StakingProductPositionService {
	s.product = product
	return s
}

// ProductId sets the productId parameter.
func (s *StakingProductPositionService) ProductId(productId string) *StakingProductPositionService {
	s.productId = &pro
    
func (s *StakingProductPositionService) Do(ctx context.Context) (*StakingProductPositions, error) {
	r := &request{
		method:   http.MethodGet,
		endpoint: "/sapi/v1/staking/position",
		secType:  secTypeSigned,
	}
	r.setParam("product", s.product)
	if s.productId != nil {
		r.setParam("productId", *s.productId)
	}
	if s.asset != nil {
		r.setParam("asset", *s.asset)
	}
	if s.current != nil {
		r.setParam("current", *s.current)
	}
	if s.size != nil {
		r.setParam("size", *s.size)
	}
	data, err := s.c.callAPI(ctx, r)
	if err != nil {
		return nil, err
	}
	res := new(StakingProductPositions)
	err = json.Unmarshal(data, res)
	if err != nil {
		return nil, err
	}
	return res, nil
}