which subsidiary of travel service serves both warsaw chopin airport and warsaw chopin airport ?
select distinct var_uri where brack_open dbr_Travel_Service_ attr_open airline attr_close dbo_subsidiary var_uri sep_dot var_uri dbo_targetAirport dbr_Warsaw_Chopin_Airport sep_dot var_uri dbo_targetAirport dbr_Warsaw_Chopin_Airport sep_dot brack_close