docs: def _validate_index(self, index):
          """Validates given index, eventually raises errors."""
          if isinstance(index, slice):
              if index.step and index.step != 1:
                  raise IndexError('Step is not allowed.')
              indexes = (index.start, index.stop)
          else:
              indexes = (index,)
          for index in indexes:
              if index is not None and index < 0:
                  raise IndexError('Negative indexes are not allowed.')