def _single_test(args):
    """Run a model on a subset of training data with a fixed test set.

    Return the output of a function specified by the last argument.

    Parameters
    ----------
    args : tuple
        Parameters and data to be passed to model.

        args[0] : int
            Increment.
        args[1] : int
            Step size. args[1] * args[0] training examples will be passed
            to the regression model.
        args[2] : array
            An n, d array of training examples.
        args[3] : list
            A list of the target values.
        args[4] : array
            An n, d array of test data.
        args[5] : list
            A list of the test target values.
        args[6] : object
            custom function for testing a predictive model.
            Must accept 4 parameters, which are args[2:5].
    """
    x = args[0]
    n = x * args[1]
    train_features = args[2]
    test = args[3]
    train_targets = args[4]
    test_targets = args[5]
    model = args[6]
    train = train_features[-n:, :]
    targets = train_targets[-n:]
    result = model(train, targets, test, test_targets)
    return result
