General Parsed Entities: Unfinished Business

General parsed entities seemed like one of the simplest parts of XML when it first appeared, easy to create and use, and more predictable than their SGML counterparts. In the years since, however, parsed entities have become a malingering reminder that DTDs are not yet dead, continuing to demonstrate periodically that XML hasn't even resolved all of the interoperability issues at the markup level. Entities solved a problem, and then became a problem themselves. The world is still waiting for a solution to the problems that entities create, though many of the solutions proposed (including my own) are either partial or create new problems. Fortunately, years of implementation experience in this area, in both SGML and XML, may provide guidance for ways forward.