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..