POLYSUB

void POLYSUB(POLY* P,POLY* Q)
{
    POLYNODE* alpha;
    POLYNODE* beta;
    int flag = 1;
    alpha = P->l->NEXT;
    beta = Q->l->NEXT;
    // checks if P is equal to Q. if flag = 1, then they are equal
    while (alpha != P->l)
    {
        if(alpha->COEFF != beta->COEFF && alpha->x != beta->x && alpha->y != beta->y && alpha->z != beta->z)
        {
            flag = 0;
            break;
        }
        alpha =  alpha->NEXT;
        beta = beta->NEXT;

    }
    if(flag == 1)
    {
        ZEROPOLY(Q);
    }
    else
    {
        beta = Q->l->NEXT;
        while(beta != Q->l)
        {
            beta->COEFF = beta->COEFF*-1;
            beta = beta->NEXT;
        }
        POLYADD(P,Q);
    }


}