Modifying data models within the template

We need to give [#includes] and subpages their own datamodels to keep 
things modular. The scheme I've come up with is to create a directive 
that can be included at the top of each template that will add that 
template's data to the top-level data model.

Two questions:

1. This is an explicit violation of the "data models are read-only" rule 
described here:

http://freemarker.sourceforge.net/docs/pgui_misc_multithreading.html

If a new datamodel is created for each page fetch, is this a problem? 
I'm running in a servlet environment. So far as I can tell, only the 
thread that is processing the page will ever see this data model.

2. When I use a HashMap as the main "root" data model, the scheme fails. 
The template says that the variables I've added to the model do not 
exist. The scheme *works* when I have the root HashMap point to an 
object, and then I replace the value of that object. There's something 
about adding a new key to the HashMap during template processing that 
the system doesn't like.

Is the template processing code playing games with my HashMap, like 
making a copy? Why?


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