I thought I understood DTD notation. However, the explain dtd contains glyphs that I have not seen before. 

The first is the " - - " after the element name and before the content model, as in:

<!ELEMENT explain - - (serverInfo, databaseInfo?, metaInfo?, indexInfo?, (recordInfo|schemaInfo)?, initInfo?)>

And, although I know what EMPTY means, I did not understand the " - 0 " in:

<!ELEMENT supports - O EMPTY>

These two glyphs imply that " 0 - " and " 0 0 " might also be glyphs.

Can someone give me an on-line reference that explains this notation?