def splitset(num_trials, skipstep=None):
    """ Split-set cross validation

    Use half the trials for training, and the other half for testing. Then
    repeat the other way round.

    Parameters
    ----------
    num_trials : int
        Total number of trials
    skipstep : int
        unused

    Returns
    -------
    gen : generator object
        the generator returns tuples (trainset, testset)
    """
    split = num_trials // 2
    a = list(range(0, split))
    b = list(range(split, num_trials))
    yield a, b
    yield b, a
