human
Solve the following python programming problem given the following code:
@pytest.fixture()
def interface():
assistant
#Here is the Python code you requested.

    """Returns the interface class for the composite.
    """
    class Base:
        counter = 0
        def add(self):
            raise NotImplemented('add    not    implemented')
        def subtract(self):
            raise NotImplemented('subtract    not    implemented')
    return Base