00050	OFF ECHO;
00075	
00100	COMMENT   These functions are concerned with generating the
00200	permutations of a list of numbers and in generating the
00300	positive and negative groups of permutations on a list of
00400	numbers.;
00500	
00600	COMMENT  isdesc u  is true if the list of numbers  u
00700	is in descending order.;
00800	
00900	ISDESC U ← NULL U OR NULL CDR U OR 
01000			(CAR U > CADR U AND ISDESC CDR U);
01100	
01200	COMMENT  next u  is the permutation following  u in order.;
01300	
01400	NEXT U ←
01450		IF NULL CDDR U THEN LIST(CADR U,CAR U)
01500		ELSE IF ISDESC CDR U THEN
01600			(LAMBDA Z; Z . SORT(CAR U . DEL(Z,CDR U)))
01700				NEXTB(CAR U,CDR U)
01800		ELSE CAR U . NEXT CDR U;
01820	
01840	NEXTB(X,U) ← 
01860		IF CAR U < X THEN NEXTB(X,CDR U)
01880		ELSE NEXTC(X,CAR U,CDR U);
01900	
02000	NEXTC(X,Y,U) ←
02100		IF NULL U THEN Y
02200		ELSE IF CAR U < X OR CAR U > Y THEN NEXTC(X,Y,CDR U)
02300		ELSE NEXTC(X,CAR U,CDR U);
02400	DEL(X,U) ←
02500		IF NULL U THEN NIL
02600		ELSE IF X = CAR U THEN CDR U
02700		ELSE CAR U . DEL(X,CDR U);