'is-a' Relationships in XML?

["stephengreenubl@gmail.com" <stephengreenubl@gmail.com>]

There seems to be no shortage of ways, thankfully, to express 'has-a' relationships with XML (even without using a schema, it seems there is an implicit 'has' relationship between parent element and a child element or atttibute). There could be said to be an implicit semantic 'has' relationship declared between 'document' an 'date' when we write <document><date>...</date>...</document>. Is there any way to express the other key relationship of 'is-a' in XML? Is this something a schema language can express? Can we say that element (or even type) A 'is-an' element (or type) B? Any plans to add this 'feature' to the XML technologies if it isn't one already? It might be a key gap to fill. I might want to somehow imply that my <invoice/> is a <document/>. Are substitution groups (with, apparently, some inadequacies) the only way to express such a relationship in XML?

Best regards

Stephen D Green