The NCBI Historical DTD, while a DTD in its own right, has been built as an overlay to the NCBI Book DTD. Therefore, there is a separate Historical DTD Module, but it redefines only <book> and <book-meta> and calls its own over-rides, then those of the Book DTD, then the defaults of the base Suite. This technique of double over-rides has the advantages that it:
This does impose a small burden on the maintainer. Whenever a change is made to NCBI Book, it will be necessary to see if the particular Parameter Entity being changed has been over-ridden in Historical. If so, that change will need to be made in parallel. While requiring such a lookup sounds like a lot of extra work and parallel maintenance is always less than best practice in fact the NCBI Historical DTD over-rides (as of this release) only six Parameter Entities from the NCBI Book DTD. It was decided that the advantages of synchronization with the NCBI Book DTD were worth the cost of checking these few.