LIBRARY= specifies EXP as the procedure input library. NOLIST suppresses the directory listing for the EXP library. The APPEND statement appends the DATA= data set, EXP.SUR, to the BASE= data set, EXP.RESULTS. FORCE causes the APPEND statement to carry out the append operation even though EXP.SUR has a variable that EXP.RESULTS does not. APPEND does not add the WT6MOS variable to EXP.RESULTS.
proc datasets library=exp nolist;
   append base=exp.results data=exp.sur force;