SIDL.xml isn't the schema, it just names the various Elements that will later be used in the other xml files. In a sense, though, it's sort of a pseudo-schema, since it does define the elements and what they can hold, but it doesn't have the various restrictions or the proper coding for a schema. Based on the files, the schema would be named EverquestData.xsd, although the current schema specification doesn't mandate that be accurate, so we really don't know.
Anyway, buy a "Free the Schema" bumper sticker, maybe we can win them over.
