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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def _check_periodic(periodic):
    '''Validate periodic input'''
    periodic = np.array(periodic)

    # If it is a matrix
    if len(periodic.shape) == 2:
        assert periodic.shape[0] == periodic.shape[1], 'periodic shoud be a square matrix or a flat array'
        return np.diag(periodic)
    elif len(periodic.shape) == 1:
        return periodic
    else:
        raise ValueError("periodic argument can be either a 3x3 matrix or a shape 3 array.")