code: def _entry_must_not_exist(df, k1, k2):
      
      count = df[(df['k1'] == k1) &
                 (df['k2'] == k2)].shape[0]
      if count > 0:
          raise AlreadyRegisteredError(
              "Option {0}.{1} already registered".format(k1, k2))
docs: Evaluate key-subkey non-existence.
  
      Checks that the key-subkey combo does not exists in the
      configuration options.