LIB "divisors.lib";
ring r=31991,(x,y,z),dp;
ideal I = y^2*z - x*(x-z)*(x+3*z);
qring Q = std(I);
divisor A = makeDivisor(ideal(x,z),ideal(1));
divisor B = makeDivisor(ideal(x,y),ideal(1));
divisor C = makeDivisor(ideal(x-z,y),ideal(1));
formaldivisor fE= makeFormalDivisor(list(list(-5,A),list(2,B)));
formaldivisor fE2= makeFormalDivisor(list(list(-5,A),list(2,C)));
formaldivisorplus(fE,fE2);
==> [1]:
==>    [1]:
==>       -10
==>    [2]:
==>       (z,x) - (1)
==> 
==> [2]:
==>    [1]:
==>       2
==>    [2]:
==>       (y,x) - (1)
==> 
==> [3]:
==>    [1]:
==>       2
==>    [2]:
==>       (y,x-z) - (1)
==>