A method or apparatus that can form and test a data block variant by flipping a selected potentially bad bit that is consecutive with 1 or 2 sequences of several potentially good bits of a received block. The fixing device heats and fixes an image on a recording material while nipping and transporting the recording material. A lower magnet is disposed in a lower region of the through hole, and a first type magnetic pole of the lower magnet is directed upward. In use the material is released from glands into the buffer solution and sedimented at the bottom of the container. The detection step primers are preferably selected so as to span the region immediately toward the 3\u2032 end from the variable nucleotide to be detected.