contextual schema change

I would like to author documents that contain within them at certain =
nodes, a different xml language such as pure xhtml or xslt+xhtml. It =
would be best if the xsi:schemaLocation could be specified to aid in =
validation and entry-help during authoring.
How is this done?

Thankyou.

Joseph Warwick,

Analyst / Programmer
CyberBillboard P/L
74/110 Sussex St
Sydney 2000

Ph: 02 92625958
Fax:02 92621195