human
Complete the below
@pytest.fixture()
def implementation(interface):

    ''''Returns an implementation of the interface'
    '''

=====
Can you provide a the Python program you requested?
assistant
#Here is the Python program you requested.
    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