def sub(self, a, b):
    '\n        field operation: substraction of polynomials > substraction of each coefficient in the appropriate field\n        '
    if (not (a.deg == b.deg)):
        a = self.reduc(a)
        b = self.reduc(b)
    c = self.neg(b)
    return self.add(a, c)
field operation: substraction of polynomials > substraction of each coefficient in the appropriate field
def sub(self, a, b):
    '\n        \n        '
    if (not (a.deg == b.deg)):
        a = self.reduc(a)
        b = self.reduc(b)
    c = self.neg(b)
    return self.add(a, c)