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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 ensure_alt_id_in_long_form(alt_id_col, long_form):
 """ Ensures alt_id_col is in long_form, and raises a ValueError if not. Parameters alt_id_col : str. Column name which denotes the column in `long_form` that contains the alternative ID for each row in `long_form`. long_form : pandas dataframe. Contains one row for each available alternative, for each observation. Returns ------- None. """

    if alt_id_col not in long_form.columns:
        msg = "alt_id_col == {} is not a column in long_form."
        raise ValueError(msg.format(alt_id_col))

    return None