def sub(self, a, b):
    '\n        field operation: substraction mod p\n        '
    return FieldElem(((a.val - b.val) % self.p), self)
field operation: substraction mod p
def sub(self, a, b):
    '\n        \n        '
    return FieldElem(((a.val - b.val) % self.p), self)