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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 requiredIfGroup(col_name, arg, dm, df, *args):
 """ Col_name is required if other columns of the group arg are present. """

    group_name = arg
    groups = set()
    columns = df.columns
    for col in columns:
        if col not in dm.index:
            continue
        group = dm.loc[col]['group']
        groups.add(group)
    if group_name in groups:
        if col_name in columns:
            return None
        else:
            return "{} column is required if column group {} is used".format(col_name, group_name)
    return None