[stringtemplate-interest] accept simple expression language

[DVD]

Hello:

I experimented with ST and really like use it for all my projects 
replacing freemarker.
The only thing I feel uncertain is that ST does not provide an option
to allow using a simple expression langauge for template author for 
customization without
requiring model developer to provide the needed data. For example,
if originally the template is like

$O.a$$O.b$

but a user wants to have only value  c = summing a and b,  something like
$O.a+O.b$

I guess in order to get this, model dev has to add property "c" in  O in 
order for author
to get.   This is a big problem if an application expects authors to 
import their own templates,
since the authors would have no way to customize it, even for some 
simple operation.
Renderer might not help so I am not sure if there is any solution for 
this scenario. I went through
the doc but did not have an example.

Thanks