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