`defpoly_mod2'
--------------

defpoly_mod2(D)
     :: Generates an irreducible univariate polynomial over GF(2).

RETURN
     univariate polynomial

D
     positive integer

   * Defined in `fff'.

   * An irreducible univariate polynomial of degree D is returned.

   * If an irreducible trinomial X^D+X^M+1 exists, then the one with
     the smallest M is returned.  Otherwise, an irreducible pentanomial
     X^D+X^M1+X^M2+X^M3+1 (M1>M2>M3 is returned.  M1, M2 and M3 are
     determined as follows: Fix M1 as small as possible. Then fix M2 as
     small as possible.  Then fix M3 as small as possible.


References
     *Note `setmod_ff': setmod_ff

