'Subtract two Rats, or a Rat and a number (reversed args).'

def __rsub__(self, other):

    if isint(other):
        other = Rat(other)
    if isRat(other):
        return Rat(((other.__num * self.__den) - (self.__num * other.__den)), (self.__den * other.__den))
    if isnum(other):
        return (other - float(self))
    return NotImplemented
