Variable structure in PCML?

Folks:

I'm trying to figure out how to implement a concept that is fairly
common ... but I'm not sure how to go about it.

What I need is a variable structure in PCML ... similar to how many of
the API's are invoked, where you give a format name, and the format
contents of a receiver field is different based on that format.

I want to do the same kind of thing with PCML ... make a call with
format information, have the called program populate that receiver field
with a pre-defined data structure based on that format, and the the java
program parse access the PCML based on what it expects the returned
variable to look like.

I thought maybe I could create structure that contained two structures,
both with an 'offset' of 0, so that both structures would start at the
beginning and overlay each other (obviously I wouldn't try to access the
wrong structure) ... but I'm thinking there may be a better way.

Any suggestions?

Thanks!

david