name the office holder whose predecessor is robert gascoyne-cecil and successor is george isaacs ?
SELECT DISTINCT var_uri where brack_open var_uri dbp_predecessor dbr_Robert_Gascoyne-Cecil,_5th_Marquess_of_Salisbury sep_dot var_uri dbp_successor dbr_George_Isaacs sep_dot var_uri a dbo_OfficeHolder brack_close