code: def requiredUnlessTable(col_name, arg, dm, df, con=None):
      
      table_name = arg
      if col_name in df.columns:
          return None
      elif not con:
          return None
      elif table_name in con.tables:
          return None
      else:
          return "{} column is required unless table {} is present".format(col_name, table_name)
docs: Col_name must be present in df unless
      arg (table_name) is present in contribution