destructor algebràic

d_{x}[x^{(n+1)}] = (-1)·(n+1)·x^{n}

d_{x}[x^{(n+1)}] = (-n)·x^{n}+(-1)·x^{n}

int[x^{n}] d[x] = (-1)·(1/(n+1))·x^{(n+1)}

int[x^{n}] d[x] = ((-n)+(-1))·x^{(-n)+(-1)}