| #### D.5.4.14 negativeformaldivisor Procedure from library `divisors.lib` (see divisors_lib). Usage: negativeformaldivisor(A); A = formaldivisor. Assume: A is a formaldivisor on X. Return: a formal divisor on X Theory: The procedure will change the signs of the coefficients. 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));
    formaldivisor fE= makeFormalDivisor(list(list(-5,A),list(2,B)));
    negativeformaldivisor(fE);
    ==> 5*( (z,x) - (1) )
    ==> -2*( (y,x) - (1) )
    ==>

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