anchor.diff:                 % filter out {element, X >= ?PLUS_INFINITY} which is invalid:
                  not (is_tuple(I) andalso element(1, I) =:= element andalso element(2, I) >= ?PLUS_INFINITY)],
       normalize_internal(List1).
  +
  +-spec tester_create_non_empty_interval([simple_interval(),...], FallbackElem::key())
  +        -> non_empty_interval().
  +tester_create_non_empty_interval(List, FallbackElem) ->
  +    I = tester_create_interval(List),
  +    case is_empty(I) of
  +        true  -> new(FallbackElem);
  +        false -> I
  +    end.
   
   -spec tester_create_continuous_interval(left_bracket(), key(), key() | ?PLUS_INFINITY, right_bracket()) -> continuous_interval().
   tester_create_continuous_interval(_LBr, Key, Key, _RBr) ->
  
positive.code: -opaque interval() :: [simple_interval()].
  -opaque invalid_interval() :: [simple_interval()].
  -type continuous_interval() :: interval().
  
  % @type interval() = [simple_interval()].
  % [] -> empty interval
  
negative.code: -spec is_empty(interval()) -> boolean().
  is_empty([]) -> true;
  is_empty(_) ->  false.
  
  %% @doc Checks whether the given interval is non-empty (mainly for tester type checker).
  -spec is_non_empty(interval()) -> boolean().
  is_non_empty(I) -> not is_empty(I).
  
  %% @doc Checks whether the given interval is covering everything.
  -spec is_all(interval()) -> boolean().
  