U.S. Patent No. 6,597,812 is directed to lossless compression and decompression with dictionary encoding (page 2).  The claim recites “maintaining a dictionary” (pages 3 and 11).  The dispute is over whether the maintaining requires retaining during the entirety of data compression unless and until the number of entries exceeds a threshold (page 11).  The creation of the dictionary is maintaining of the dictionary (pages 12-13).  The specification discloses dynamic maintenance and updating (page 13).  The “comprising” language does not result in reading more into the maintaining limitation (pages 13-14).  The specification teaching of resetting the dictionary based on the threshold is only an embodiment and does not indicate that the reset must be part of maintaining (page 14).

Hindsight:  A dependent claim included the reset and threshold language, but was not subject to invalidity.  In general, independent claims should be drafted without restrictions on the broad terms used.  The result here reflects this approach as the non-invalidated dependent claim included a limitation not in the prior art while maintaining was broadly interpreted to not require reset and threshold.