assuming many 'sub vars' to one 'main var', the interface you've created is essentially an alias for /hadd and $hget() ;) but i've always liked the idea of creating utilities for other scripters, so i look forward to future submissions made in the same vein.

% [ $+ [ $1 ] ] [ $+ [ . ] ] [ $+ [ $2 ] ]

there's a lot going on behind the scenes there, and it's worth noting that the above can be simplified to:

% [ $+ [ $1 ] $+ . $+ [ $2 ] ]

the simplification makes the pattern easier to understand, should you try to do so. evaluation brackets are fascinating, but as pball showed, their use in this context has been superseded by $eval().