`gcd', `gcdz'
-------------

gcd(POLY1,POLY2[,MOD])

gcdz(POLY1,POLY2)
     :: The polynomial greatest common divisor of POLY1 and POLY2.

RETURN
     polynomial

POLY1 POLY2
     polynomial

MOD
     prime

   * Functions `gcd()' and `gcdz()' return the greatest common divisor
     (GCD) of the given two polynomials.

   * Function `gcd()' returns an integral polynomial GCD over the
     rational number field.  The coefficients are normalized such that
     their GCD is 1.  It returns 1 in case that the given polynomials
     are mutually prime.

   * Function `gcdz()' works for arguments of integral polynomials, and
     returns a polynomial GCD over the integer ring, that is, it
     returns `gcd()' multiplied by the contents of all coefficients of
     the two input polynomials.

   * `gcd()' computes the GCD over GF(MOD) if MOD is specified.

   * Polynomial GCD is computed by an improved algorithm based on
     Extended Zassenhaus algorithm.

   * GCD over a finite field is computed by PRS algorithm and it may
     not be efficient for large inputs and co-prime inputs.

     [0] gcd(12*(x^2+2*x+1)^2,18*(x^2+(y+1)*x+y)^3);
     x^3+3*x^2+3*x+1
     [1] gcdz(12*(x^2+2*x+1)^2,18*(x^2+(y+1)*x+y)^3);
     6*x^3+18*x^2+18*x+6
     [2] gcd((x+y)*(x-y)^2,(x+y)^2*(x-y));
     x^2-y^2
     [3] gcd((x+y)*(x-y)^2,(x+y)^2*(x-y),2);
     x^3+y*x^2+y^2*x+y^3

References
     *Note `igcd igcdcntl': igcd igcdcntl.

