Transformations between different document types

Hi,

 Forige me if I seem like a confused being - but yeah, some help on
this would be real good.

I need to define XSL Transformations between different document types.
Lets say one of them between an invoice and a po, another one between
some RN and OAG documents, the list goes on.

I can do that, with some effort.

How do I expose a web service that can transform one doc to another.
There could be a portType called transformPortType, that holds
operations that have the following in and out parameters.
There will be obviously one parameter that holds a reference to the
XSL file [lets say in the form of a URL String]. It also takes in a
Source Node. It would need to return the Document for the transformed
result.

How would the WSDL for the web service look like ? Do I need to define
separate operations for transformations on different document types,
or have a single transformation operation that transforms from anyType
to anyType ?
What is the usual practice ?

Hope I have been clear enough in the Problem statement.

rgds,
Garfield.