AS=PFSDIP(RL,A)
$(POLYNOMIAL FROM SPECIAL DISTRIBUTIVE POLYNOMIAL. A IS A DISTRIBUTIVE
POLYNOMIAL IN RL VARIABLES, RL NON-NEGATIVE.  AS IS THE RESULT OF
CONVERTING A TO A RECURSIVE POLYNOMIAL IN RL VARIABLES.)
     SAFE AL1,EL1,E1.
(1)  $(A=0.)  IF A=0 THEN ( AS=0.  RETURN ).
(2)  $(RL=0.)  IF RL=0 THEN ( AS=FIRST(A).  RETURN ).
(3)  $(RECURSION.)  AS=().  AP=A.  RLP=RL-1.  REPEAT
     (   EL=SDIPDG(RL,AP).  A1=().  REPEAT (  EL1=SDIPDG(RL,AP).
     IF EL1=EL THEN ( ADV2(AP.AL1,E1,AP).  IF EL1 GT 0 THEN E1=RED2(E1).
     A1=COMP2(E1,AL1,A1) )  ) UNTIL EL1 LT EL OR AP=().  A1=INV(A1).
     AS1=PFSDIP(RLP,A1).  AS=COMP2(AS1,EL,AS)   ) UNTIL AP=().
     AS=INV(AS).  RETURN..