code: public function merchant($id = null) {
  
      if ($id == null) {
        $id = $this->account_details['merchant_id'];
      }
  
      return GoCardless_Merchant::find_with_client($this, $id);
  
    }
docs: Returns the merchant associated with the client's access token
  
  @param string $id The id of the merchant to fetch
  
  @return object The merchant object