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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_col_items(mapping):
 """Get all column items to format, including both keys of `mapping` and MultiIndex levels if any. """

    from .variable import IndexVariable

    col_items = []
    for k, v in mapping.items():
        col_items.append(k)
        var = getattr(v, 'variable', v)
        if isinstance(var, IndexVariable):
            level_names = var.to_index_variable().level_names
            if level_names is not None:
                col_items += list(level_names)
    return col_items