{{{#!python
class Expr(object):
    def __init__(self, a, op=None, b=None):
        self.a, self.op, self.b = a, op, b
    def __repr__(self):
        if self.op:
            if self.b:
                return '(%r %s %r)' % (self.a, self.op, self.b)
            else:
                return '(%s%r)' % (self.op, self.a)
        return self.a
    def __add__(self, other):
        return Expr(self, '+', other)
    def __sub__(self, other):
        return Expr(self, '-', other)
    def __mul__(self, other):
        return Expr(self, '*', other)
    def __div__(self, other):
        return Expr(self, '/', other)
    def __neg__(self, other):
        return Expr(self, '-')
    def __pos__(self, other):
        return Expr(self, '+')
}}}