code: func (c *Client) ExchangePublicToken(publicToken string) (resp ExchangePublicTokenResponse, err error) {
  	if publicToken == "" {
  		return resp, errors.New("/item/public_token/exchange - public token must be specified")
  	}
  
  	jsonBody, err := json.Marshal(exchangePublicTokenRequest{
  		ClientID:    c.clientID,
  		Secret:      c.secret,
  		PublicToken: publicToken,
  	})
  
  	if err != nil {
  		return resp, err
  	}
  
  	err = c.Call("/item/public_token/exchange", jsonBody, &resp)
  	return resp, err
  }
docs: // ExchangePublicToken exchanges a public token for an access token.
  // See https://plaid.com/docs/api/#exchange-token-flow.