I need your help here.

I’ve been working on my web template engine and I can’t decide exactly what sort of things should be allowed in templates. I think most people agree that these things should be available:

1. Property Access
2. if/else/else if Branching
3. Iteration over loops
4. Formatting
5. Math operations (think if (foo > 10 + bar))

It gets a little foggier here:

1. Method Invocation
2. Variable Declaration
3. Variable Setting
4. Method Declaration

After that its pretty much full on double rainbow access to the underlying language.

{{ 
     if (count > foobar ()) {
         Response.Write ("foo");
     }
}}

I think most people agree that there should be a line drawn and code should be kept out of the templates, but where do you feel the template engine should draw the line?