'Make a pretty division; stacked or slashed.'

def __div__(self, den, slashed=False):

    if slashed:
        raise NotImplementedError("Can't    do    slashed    fraction    yet")
    num = self
    if (num.binding == prettyForm.DIV):
        num = stringPict(*num.parens())
    if (den.binding == prettyForm.DIV):
        den = stringPict(*den.parens())
    if (num.binding == prettyForm.NEG):
        num = num.right('    ')[0]
    return prettyForm(binding=prettyForm.DIV, *stringPict.stack(num, stringPict.LINE, den))
