n=SDIPDG(r,A)
[Special distributive polynomial degree. A is a distributive
polynomial in r variables, r>=0.  n is the degree of A in its
main variable.]
     safe SDIPDG.
(1)  if r==0|A==0 then n=0 else { E=SECOND(A);  if E==() then
     n=0 else if FIRST(E)<r then n=0 else n=SECOND(E) };
     return..