DTDs, by their nature, contain many complex, interrelated elements. Whenever you make a change to the DTD, it's always wise to use a validating parser to double-check your work. A parser like nsgmls from James Clark's SP can identify elements (attributes, parameter entities) that are declared but unused, as well as ones that are used but undeclared.
A comprehensive test can be accomplished with nsgmls using the -wall option. Create a simple test document and run:
nsgmls |
If you run the preceding command over DocBook V3.1, you'll discover one warning generated by the DTD:
nsgmls:I: SP version "1.3" nsgmls:cals-tbl.dtd:314:37:W: content model is mixed but does not allow #PCDATA everywhere |
This is not truly an error in the DTD, and can safely be ignored. The warning is caused by "pernicious mixed content" in the content model of DocBook's Entry element. See the Entry reference page for a complete discussion.