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

    """Returns an implementation of the interface
    """
    class Implementation(interface, ):
        def __init__(self, value):
            self.value = value
        def add(self):
            interface.counter += self.value
        def subtract(self):
            interface.counter -= self.value
    return Implementation