'Make a pretty addition.
Addition of negative numbers is simplified.'

def __add__(self, *others):

    arg = self
    if (arg.binding > prettyForm.NEG):
        arg = stringPict(*arg.parens())
    result = [arg]
    for arg in others:
        if (arg.binding > prettyForm.NEG):
            arg = stringPict(*arg.parens())
        if (arg.binding != prettyForm.NEG):
            result.append('    +    ')
        result.append(arg)
    return prettyForm(binding=prettyForm.ADD, *stringPict.next(*result))
