Semantic MediaWiki

I'm working with SMW 1.0, using it to track projects we're working on
at work.  We have "tasks" which "belong to" a project and each task
has an annotation for "estimated remaining" time.

What I'd like to do is sum the estimated remaining annotations for all
the tasks belonging to a project.  I can write the query to show them
in a list, and tried to use a template to output them like:

+ 2 + 1 + 3

thinking that I could pass that into {{#expr: ...}}.  Unfortunately it
doesn't seem that the nesting works quite right; #expr complains:

Expression error: Unrecognised punctuation character "["

Anyone have any suggestions on how to handle this?

Thanks,

Nathan