anchor.diff:        verificationResult.called(2);
       });
   
  -    test('should use returned data (first element - training data, second '
  -        'element - testing data) from the callback for data preprocessing '
  -        'while evaluating a predictor', () {
  +    test('should take the first element as train samples from data '
  +        'preprocessing callback response while evaluating a predictor', () {
         final allObservations = DataFrame(
           <Iterable<num>>[
             [1, 1, 1, 1],
  
positive.code:             [111, 888, 999, 222],
              [301, 403, 501, 607],
            ], headerExists: false),
            DataFrame(<Iterable<num>>[
              [44, 55, 66, 11],
              [29, 22, 11,  0],
  
negative.code:             [1, 2, 3, 4],
              [7, 8, 9, 0],
            ], headerExists: false),
            DataFrame(<Iterable<num>>[[]], headerExists: false),
          ],
          1: [
            DataFrame(<Iterable<num>>[
              [100, 200, 300, 400],
              [117, 118, 119, 110],
            ], headerExists: false),
            DataFrame(<Iterable<num>>[[]], headerExists: false),
          ],
          2: [
            DataFrame(<Iterable<num>>[
  