[boost] [proto] How to convert a constant into a proto terminal?

I fear this could be the first of a stream of feeble questions as I try to provoke Proto into life: As the docs say, "Once we have some Proto terminals, expressions involving those terminals build expression trees for us" ... "As long as one operand is a Proto expression, the result of the operation is a tree node representing that operation". In the calculator example, if I write 10 + 20 + _1 then the 10+20 portion is not a proto expression, which happens to be useful behaviour in this case. But if I wanted to make the 10+20 subexpression a proto expression, I'd need to somehow taint or cast 10. How would I do that? The closest I've spotted in the docs is make_expr(); is this what I need? Phil.