class Formula(object):
    def __init__(self, o1, o2, o):
        self.o1=o1
        self.o2=o2
        self.o=o
        self.is_oper=False
    def __repr__(self):
        return "(%s%s%s)" %(self.o1, self.o, self.o2)
    __str__=__repr__
    @classmethod
    def from_postfix(cl, string):
        atoms=list(map(Atom, string))
        while len (atoms)>1:
            for i,a in enumerate(atoms):
                if not a.is_oper:
                    continue
                else:
                    if a.is_unar:
                        a.parse_unar (atoms,i)
                    else:
                        a.parse_postfix(atoms, i)
                break
        return atoms[0]