NTHCDR

[JONL at MIT-MC (Jon L White)]

1) In *RSET mode, NTHCDR fails to check the first arg for FIXNUMness.
   Should probably also check that it is less than about 2^18 or so.
2) I would like to replace my own function NCDR with NTHCDR, but the 
   latter is too slow (even though the former is compiled LISP!)
   Timings:  (NCDR l n)		12.us + 1.80*<n-1>us
	     (NTHCDR n l)	 8.us + 3.86*<n-1>us
   How about trying the loop-unfolding trick, and inserting just before
   the label "NTHCD1" this code
   NTHCD0:	CAIGE R,4
		  JRST NTHCD2
   REPEAT 4,	HRRZ A,(A)
		SUBI R,4
		JRST NTHCD0
   NTHCD2:	JUMPLE R,CPOPJ
   NTHCD0: 	. . .