def requiredOneInGroup(col_name, group, dm, df, *args):
    """
    If col_name is present in df, the group validation is satisfied.
    If not, it still may be satisfied, but not by THIS col_name.
    If col_name is missing, return col_name, else return None.
    Later, we will validate to see if there is at least one None (non-missing)
    value for this group.
    """
    if col_name in df.columns:
        # if the column name is present, return nothing
        return None
    else:
        # if the column name is missing, return column name
        return col_name