| #### D.5.4.13 formaldivisorplus Procedure from library `divisors.lib` (see divisors_lib). Usage: formaldivisorplus(A ,B); A + B; A = formaldivisor, B = formaldivisor. Assume: A and B are formal divisors on X. Return: a formal divisor on X Theory: The procedure will add the formal sums. Example:
    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)
    ==>

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