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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 get_axis_variables(ds):
    '''
    Returns a list of variables that define an axis of the dataset

    :param netCDF4.Dataset ds: An open netCDF4 Dataset
    '''
    axis_variables = []
    for ncvar in ds.get_variables_by_attributes(axis=lambda x: x is not None):
        axis_variables.append(ncvar.name)
    return axis_variables