import zope.interface

class IExpression(zope.interface.Interface):
    def evaluate(self):
        """ Retrns the value of the expression """


class Concatination(IExpression):
    def __init__(self, left, right):
        self.left = left
        self.right = right

    def evaluate(self):
        return self.left.evaluate() + self.right.evaluate()


class Letter:
    def __init__(self, l):
        self.l = l;

    def evaluate(self):
        return self.l

c = Concatination(Letter('a'),Letter('b'))
print(c.evaluate())

@zope.interface.implementer(IExpression)
class Concatination:
    ...

class IExpression(zope.interface.Interface):
    def evaluate():
        """ Return the value of the expression."""