>>> from sympy.core.exprtools import Factors
        >>> from sympy.abc import x, y, z
        >>> a = Factors((x*y**2).as_powers_dict())
        >>> b = Factors((x*y/z).as_powers_dict())
        >>> a.rem(b)
        Factors({z: -1})
        >>> a.rem(a)
        Factors({})
        r