code: def requiredUnless(col_name, arg, dm, df, *args):
      
      # if column name is present, no need to check if it is required
      if col_name in df.columns:
          return None
      arg_list = arg.split(",")
      arg_list = [argument.strip('"') for argument in arg_list]
      msg = ""
      for a in arg_list:
          # ignore validations that reference a different table
          if "." in a:
              continue
          if a not in df.columns:
              msg += "{} column is required unless {} is present.  ".format(col_name, a)
      if msg:
          return msg
      else:
          return None
      return None
docs: Arg is a string in the format "str1, str2, ..."
      Each string will be a column name.
      Col_name is required in df unless each column from arg is present.