def _coallate(list_a, list_b):
    """ Given: _coallate([ '1', '2', '3', '4'], ['a', 'b', 'c', 'd'])
        Return: ['1', 'a', '2', 'b', '3', 'c', '4', 'd']
    """
    return [i for sub in zip(list_a, list_b) for i in sub]