Next Combination
CALL sla_COMBN (NSEL, NCAND, LIST, J)
NSEL  I  number of items (subset size) 


NCAND  I  number of candidates (set size) 
LIST  I(NSEL)  latest combination, LIST(1)=0 to initialize 
J  I  status: 
 

 $$1 = illegal NSEL or NCAND 



 0 = OK 



 $+$1 = no more combinations available 
LIST(1)=1, LIST(2)=2, …, LIST(NSEL)=NSEL
LIST(1)=NCAND, LIST(2)=NCAND$$1,
…,
LIST(NSEL)=NCAND$$NSEL+1