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

From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2008-03-06 15:12:53

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.